Oxidation catalyst for treating the exhaust gas of a compression ignition engine

ABSTRACT

An exhaust system for a compression ignition engine comprising an oxidation catalyst for treating carbon monoxide (CO) and hydrocarbons (HCs) in exhaust gas from the compression ignition engine, wherein the oxidation catalyst comprises: a platinum group metal (PGM) component selected from the group consisting of a platinum (Pt) component, a palladium (Pd) component and a combination thereof; an alkaline earth metal component; a support material comprising a modified alumina incorporating a heteroatom component; and a substrate, wherein the platinum group metal (PGM) component, the alkaline earth metal component and the support material are disposed on the substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 14/086,019, filed Nov. 21, 2013, which claimspriority benefit to Great Britain Patent Application No. 1220912.8 filedon Nov. 21, 2012, and to U.S. Provisional Patent Application No.61/728,834 filed on Nov. 21, 2012, and to Great Britain PatentApplication No. 1308934.7 filed on May 17, 2013, all of which areincorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to an exhaust system for a compression ignitionengine that comprises an oxidation catalyst, particularly a dieseloxidation catalyst, and to a vehicle comprising the exhaust system. Theinvention also relates to a process of preparing the oxidation catalyst,to the oxidation catalyst itself and its uses. The invention furtherrelates to a method of treating an exhaust gas from a compressionignition engine.

BACKGROUND TO THE INVENTION

Generally, there are four classes of pollutant that are legislatedagainst by inter-governmental organisations throughout the world: carbonmonoxide (CO), unburned hydrocarbons (HCs), oxides of nitrogen (NO_(x))and particulate matter (PM). As emissions standards for permissibleemission of pollutants in exhaust gases from vehicular engines becomeprogressively tightened, there is a need to provide improved catalyststhat are able to meet these standards and which are cost-effective.

For compression ignition engines, such as diesel engines, an oxidationcatalyst (known as a diesel oxidation catalyst (DOC)) is typically usedto treat the exhaust gas produced by such engines. Diesel oxidationcatalysts generally catalyse the oxidation of (1) carbon monoxide (CO)to carbon dioxide (CO₂), and (2) HCs to carbon dioxide (CO₂) and water(H₂O). Exhaust gas temperatures for compression ignition engines, suchas diesel engines particularly for light-duty diesel vehicles, arerelatively low (e.g. about 400° C.) and so one challenge is to developdurable catalyst formulations with low “light-off” temperatures.

The activity of oxidation catalysts, such as DOCs, is often measured interms of its “light-off” temperature, which is the temperature at whichthe catalyst starts to perform a particular catalytic reaction orperforms that reaction to a certain level. Normally, “light-off”temperatures are given in terms of a specific level of conversion of areactant, such as conversion of carbon monoxide. Thus, a T₅₀ temperatureis often quoted as a “light-off” temperature because it represents thelowest temperature at which a catalyst catalyses the conversion of areactant at 50% efficiency.

EP 2000639 describes a method for increasing the temperature of anexhaust gas from an internal combustion engine, particularly a dieselengine. EP 2000639 is concerned with increasing the temperature of anexhaust gas because it can assist the regeneration of a particulatefilter placed downstream from the catalyst. The method described in EP2000639 involves introducing hydrocarbon (HC) in an amount of from 1,000to 40,000 ppm by volume, as converted to methane, to the exhaust gasupstream of a catalyst. The catalyst is obtained by supporting acatalytically active component (A) consisting of (a) platinum, (b) anoxide of at least one metal selected from the group consisting ofmagnesium, an alkaline earth metal and an alkali metal, and (c) at leastone member selected from the group of palladium and rhodium, on arefractory inorganic oxide powder (B), and supporting the inorganicoxide on a refractory three-dimensional structure body.

SUMMARY OF THE INVENTION

Catalysts that are used to oxidise carbon monoxide (CO), hydrocarbons(HCs) and sometimes also oxides of nitrogen (NO_(x)) in an exhaust gasemitted from a compression ignition engine generally comprise at leastone platinum group metal, such as platinum or palladium. Platinum ismore active than palladium at catalysing the oxidation of CO and HCs inthe exhaust gas from a compression ignition engine and the inclusion ofpalladium in such catalysts was generally avoided because of itssusceptibility to poisoning by sulphur. However, the use of ultra-lowsulphur fuels, the relative cost of palladium to platinum, andimprovements in catalyst durability that can be obtained by inclusion ofpalladium have resulted in catalyst formulations comprising palladium,especially formulations comprising both palladium and platinum, becomingfavoured.

Even though, in general, the cost of palladium has historically beenlower than that of platinum, both palladium and platinum are expensivemetals. Oxidation catalysts that show improved catalytic activitywithout increasing the total amount of platinum and palladium, or thatshow similar catalytic activity to existing oxidation catalysts with alower amount of platinum and palladium, are desirable. In a firstaspect, the invention provides an exhaust system for a compressionignition engine comprising an oxidation catalyst typically for treatingcarbon monoxide (CO) and hydrocarbons (HCs) in exhaust gas from thecompression ignition engine, wherein the oxidation catalyst comprises: aplatinum group metal (PGM) component selected from the group consistingof a platinum (Pt) component, a palladium (Pd) component and acombination thereof; an alkaline earth metal component; a supportmaterial comprising a modified alumina incorporating a heteroatomcomponent; and a substrate, wherein the platinum group metal (PGM)component, the alkaline earth metal component and the support materialare disposed on the substrate.

The inventors have surprisingly found that an oxidation catalyst havingadvantageous activity can be obtained when a combination of (i) analkaline earth metal component and (ii) an alumina support material thathas been modified to include a heteroatom component, is included in acatalyst formulation comprising at least one of platinum and palladium.Such catalysts have been found to have excellent low temperature COoxidation activity. The catalysts are particularly effective inconverting relatively high levels of CO in exhaust gas produced by thecompression ignition engine, particularly at temperatures below 250° C.The catalysts may also show good oxidation activity towards HCs,particularly unsaturated HCs such as alkenes, at low temperatures. Therelatively low temperature oxidation activity of the catalyst renders itparticularly suitable for use in combination with other emissionscontrol devices in an exhaust system. In particular, the oxidationcatalyst is able to oxidise nitrogen oxide (NO) to nitrogen dioxide(NO₂), which can be advantageous when the oxidation catalyst is upstreamof a selective catalytic reduction (SCR) catalyst or a selectivecatalytic reduction filter (SCRF) catalyst.

The initial oxidative activity of a freshly prepared oxidation catalystoften deteriorates until the catalyst reaches an aged state. Repeatedexposure of the oxidation catalyst to hot exhaust gas can causesintering and/or alloying of the platinum group metal (PGM) componentsof the catalyst until it reaches an aged state. This deterioration inactivity can be problematic, particularly when pairing the oxidationcatalyst with one or more other emissions control devices in an exhaustsystem. The oxidation catalyst of the invention may have stable activitytoward oxidising nitrogen oxide (NO) to nitrogen dioxide (NO₂) (i.e. the“fresh” oxidative activity of the catalyst toward NO is the same orsimilar to the “aged” oxidative activity of the catalyst). This isparticularly advantageous, even when the amount of NO oxidation may berelatively low, for exhaust systems where the oxidation catalyst iscombined with a selective catalytic reduction (SCR) catalyst or aselective catalytic reduction filter (SCRF) catalyst because an exhaustgas having a stable ratio of NO:NO₂ can be passed into the SCR or SCRFcatalyst.

A second aspect of the invention relates to a vehicle comprising acompression ignition engine and an exhaust system for the compressionignition engine, wherein the exhaust system comprises an oxidationcatalyst, and wherein the oxidation catalyst comprises: a platinum groupmetal (PGM) component selected from the group consisting of a platinum(Pt) component, a palladium (Pd) component and a combination thereof; analkaline earth metal component; a support material comprising a modifiedalumina incorporating a heteroatom component; and a substrate, whereinthe platinum group metal (PGM) component, the alkaline earth metalcomponent and the support material are disposed on the substrate.

In a third aspect, the invention relates to an oxidation catalyst and toits use for treating an exhaust gas from a compression ignition engine,such as a diesel engine. The oxidation catalyst comprises: a platinumgroup metal (PGM) component selected from the group consisting of aplatinum (Pt) component, a palladium (Pd) component and a combinationthereof; an alkaline earth metal component; a support materialcomprising a modified alumina incorporating a heteroatom component; anda substrate, wherein the platinum group metal (PGM) component, thealkaline earth metal component and the support material are disposed onthe substrate. The invention generally relates to the use of theoxidation catalyst to oxidise carbon monoxide (CO) in an exhaust gasfrom a compression ignition engine. In particular, the invention relatesto use of the oxidation catalyst to oxidise carbon monoxide (CO),hydrocarbons (HCs) and optionally oxides of nitrogen (NO_(x)), such asnitrogen oxide (NO), in an exhaust gas from a compression ignitionengine.

In a fourth aspect, the invention provides a method of treating anexhaust gas from a compression ignition engine, which method comprisescontacting the exhaust gas with an oxidation catalyst, wherein theoxidation catalyst comprises: a platinum group metal (PGM) componentselected from the group consisting of a platinum (Pt) component, apalladium (Pd) component and a combination thereof; an alkaline earthmetal component; a support material comprising a modified aluminaincorporating a heteroatom component; and a substrate, wherein theplatinum group metal (PGM) component, the alkaline earth metal componentand the support material are disposed on the substrate. The method isgenerally a method of treating carbon monoxide (CO), preferably treatingcarbon monoxide (CO), hydrocarbons (HCs) and optionally oxides ofnitrogen (NO_(x)), such as nitrogen oxide (NO), in an exhaust gas from acompression ignition engine.

A fifth aspect of the invention relates to a process of preparing anoxidation catalyst, which process comprises disposing a platinum groupmetal (PGM) component, an alkaline earth metal component, and a supportmaterial onto a substrate, wherein the platinum group metal (PGM)component is selected from the group consisting of a platinum (Pt)component, a palladium (Pd) component and a combination thereof, and thesupport material comprises a modified alumina incorporating a heteroatomcomponent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a histogram showing measurements of carbon monoxide“light-off” temperatures (° C.) at 50% conversion (CO T₅₀) for catalystformulations comprising varying amounts of platinum and palladium. Thex-axis shows the % by weight of platinum in relation to the totalcontent of platinum group metal in the formulation. At each point on thex-axis, the first bar (from the left) represents a sample containingstandard alumina and no barium, the second bar represents a samplecontaining alumina and barium, the third bar represents a samplecontaining a modified alumina and no barium, and the fourth barrepresents a sample containing a modified alumina and barium.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to an oxidation catalyst comprising an alkalineearth metal component. It has surprisingly been found that a catalysthaving advantageous oxidising activity, particularly a low CO T₅₀, canbe obtained for catalyst formulations comprising an alkaline earth metalcomponent and a modified alumina incorporating a heteroatom component.

Typically, the alkaline earth metal component comprises magnesium (Mg),calcium (Ca), strontium (Sr), barium (Ba) or a combination of two ormore thereof. It is preferred that the alkaline earth metal componentcomprises calcium (Ca), strontium (Sr), or barium (Ba), more preferablystrontium (Sr) or barium (Ba), and most preferably the alkaline earthmetal component comprises barium (Ba).

Generally, the alkaline earth metal component comprises a singlealkaline earth metal selected from the group consisting of (Mg), calcium(Ca), strontium (Sr) and barium (Ba). Preferably, the alkaline earthmetal component comprises a single alkaline earth metal selected fromthe group consisting of calcium (Ca), strontium (Sr) and barium (Ba),more preferably strontium (Sr) and barium (Ba), and most preferably thealkaline earth metal component comprises a single alkaline earth metalthat is barium (Ba).

Typically, the amount of the alkaline earth metal component is 0.07 to3.75 mol ft⁻³, particularly 0.1 to 3.0 mol ft⁻³, more particularly 0.2to 2.5 mol ft⁻³ (e.g. 0.25 to 1.0 mol ft⁻³), such as 0.3 to 2.25 molft⁻³, especially 0. 0.35 to 1.85 mol ft⁻³, preferably 0.4 to 1.5 molft⁻³, even more preferably 0.5 to 1.25 mol ft⁻³. Without wishing to bebound by theory, it is believed that the number of alkaline earth metalatoms that are present contributes to the advantageous activity of thecatalyst and that this activity “levels-off” once the number of alkalineearth metal atoms has reached a certain amount. Too much of the alkalineearth metal component may affect the HC and NO oxidation performance ofthe catalyst. If there are an insufficient number of alkaline earthmetal atoms, then the advantageous activity may not be obtained.

Generally, the total amount of the alkaline earth metal component is 10to 500 g ft⁻³ (e.g. 60 to 400 g ft⁻³ or 10 to 450 g ft⁻³), particularly20 to 400 g ft⁻³, more particularly 35 to 350 g ft⁻³, such as 50 to 300g ft⁻³, especially 75 to 250 g ft⁻³.

The oxidation catalyst, in general, comprises an amount of the alkalineearth metal component of 0.1 to 20% by weight, preferably 0.5 to 17.5%by weight, more preferably 1 to 15% by weight, and even more preferably1.5 to 12.5% by weight. The amount of the alkaline earth metal componentmay be from 1.0 to 8.0% by weight, such as 1.5 to 7.5% by weight,particularly 2.0 to 7.0% by weight (e.g. 2.5 to 6.5% by weight or 2.0 to5.0% by weight). The amount of the alkaline earth metal component may befrom 5.0 to 17.5% by weight, such as 7.5 to 15% by weight, particularly8.0 to 14% by weight (e.g. 8.5 to 12.5% by weight or 9.0 to 13.5% byweight).

Typically, the ratio of the total mass of the alkaline earth metalcomponent to the total mass of the platinum group metal (PGM) componentis 0.25:1 to 20:1 (e.g. 0.3:1 to 20:1). It is preferred that the ratioof the total mass of the alkaline earth metal component to the totalmass of the platinum group metal (PGM) component is 0.5:1 to 17:1, morepreferably 1:1 to 15:1, particularly 1.5:1 to 10:1, still morepreferably 2:1 to 7.5:1, and even more preferably 2.5:1 to 5:1. When aplatinum (Pt) component is present, then preferably the total mass ofthe alkaline earth component is greater than the total mass of theplatinum (Pt) component.

The support material typically comprises, or consists essentially of, amodified alumina incorporating a heteroatom component. The heteroatomcomponent that is incorporated into the alumina generally changes thechemical characteristics, physical structure and/or physical propertiesof the material in comparison to alumina itself, and generally also incomparison to a mixture of alumina with the heteroatom component. It isthought that the presence of the heteroatom component modifies theinteraction of the alumina with the alkaline earth component. Themodified alumina is typically alumina present in, or originating from,the gamma form (i.e. γ-alumina).

Typically, the heteroatom component comprises an element selected fromthe group consisting of a lanthanide and any one of Groups 1 to 14 ofthe Periodic Table (the IUPAC nomenclature for numbering the Groups ofthe Periodic Table is used herein, such that Group 1 comprises thealkali metals, Group 4 comprises Ti, Zr etc., and Group 14 comprises C,Si etc.). Preferably, the heteroatom component comprises an elementselected from Group 2 (e.g. Mg, Ca, Sr or Ba), Group 4 (e.g. Ti or Zr),Group 14 (e.g. Si) of the Periodic Table and a lanthanide (e.g. La orCe), such as an element selected from Group 4 (e.g. Ti or Zr), Group 14(e.g. Si) of the Periodic Table and a lanthanide (e.g. La or Ce). Theheteroatom component can be an element, ion or a compound, but it is notalumina and, preferably, it is not a constituent element or ion ofalumina (e.g. oxygen, O²⁻, aluminium or Al³⁺).

The modified alumina incorporating a heteroatom component generallycomprises, or consists essentially of, an alumina doped with aheteroatom component, an alkaline earth metal aluminate or a mixturethereof. It is preferred that the modified alumina incorporating aheteroatom component comprises, or consists essentially of, an aluminadoped with a heteroatom component or an alkaline earth metal aluminate.

When the modified alumina incorporating a heteroatom component isalumina doped with a heteroatom component, then typically the heteroatomcomponent comprises silicon, magnesium, barium, lanthanum, cerium,titanium, or zirconium or a combination of two or more thereof. Theheteroatom component may comprises, or consist essentially of, an oxideof silicon, an oxide of magnesium, an oxide of barium, an oxide oflanthanum, an oxide of cerium, an oxide of titanium or an oxide ofzirconium. Preferably, the heteroatom component comprises, or consistsessentially of, silicon, magnesium, barium, or cerium, or an oxidethereof, particularly silicon, or cerium, or an oxide thereof. Morepreferably, the heteroatom component comprises, or consists essentiallyof, silicon, magnesium, or barium, or an oxide thereof; particularlysilicon, or magnesium, or an oxide thereof; especially silicon or anoxide thereof.

Examples of alumina doped with a heteroatom component include aluminadoped with silica, alumina doped with magnesium oxide, alumina dopedwith barium or barium oxide, alumina doped with lanthanum oxide, oralumina doped with ceria, particularly alumina doped with silica,alumina doped with lanthanum oxide, or alumina doped with ceria. It ispreferred that the alumina doped with a heteroatom component is aluminadoped with silica, alumina doped with barium or barium oxide, or aluminadoped with magnesium oxide. More preferably, the alumina doped with aheteroatom component is alumina doped with silica or alumina doped withmagnesium oxide. Even more preferably, the alumina doped with aheteroatom component is alumina doped with silica. Alumina doped with aheteroatom component can be prepared using methods known in the art or,for example, by a method described in U.S. Pat. No. 5,045,519.

Typically, the alumina doped with a heteroatom component comprises 0.5to 45% by weight of the heteroatom component, preferably 1 to 40% byweight of the heteroatom component, more preferably 1.5 to 30% by weightof the heteroatom component, particularly 2.5 to 25% by weight of theheteroatom component.

When the alumina doped with a heteroatom component comprises, orconsists essentially of, alumina doped with silica, then the alumina isdoped with silica in an amount of 0.5 to 45% by weight, preferably 1 to40% by weight, more preferably 1.5 to 30% by weight (e.g. 1.5 to 10% byweight), particularly 2.5 to 25% by weight, more particularly 3.5 to 20%by weight (e.g. 5 to 20% by weight), even more preferably 4.5 to 15% byweight.

When the alumina doped with a heteroatom component comprises, orconsists essentially of, alumina doped with magnesium oxide, then thealumina is doped with magnesium in an amount as defined above or anamount of 5 to 30% by weight, preferably 10 to 25% by weight.

If the heteroatom component comprises, or consists essentially of, analkaline earth metal, then generally the oxidation catalyst comprises analkaline earth metal component that is separate to, or is not part of,the modified alumina incorporating a heteroatom component. Thus, theoxidation catalyst includes an alkaline earth metal component inaddition to any alkaline earth metal that may be present in the modifiedalumina.

Generally, when the heteroatom component comprises, or consistsessentially of, an alkaline earth metal, then preferably the alkalineearth metal component is different to the heteroatom component. It ispreferred that the heteroatom component and the alkaline earth metalcomponent comprise different alkaline earth metals.

If the heteroatom component of the modified alumina comprises analkaline earth metal, such as when it is a dopant in the alumina dopedwith a heteroatom component or when it is part of the alkaline earthmetal aluminate, then the amount of the “alkaline earth metal component”does not include the amount of any alkaline earth metal that is presentas part of the modified alumina. Similarly, the amount of heteroatomcomponent does not include the amount of the alkaline earth metalcomponent that is present. It is possible to control the amounts of eachcomponent during manufacture of the oxidation catalyst.

The term “alkaline earth metal aluminate” generally refers to a compoundof the formula MAl₂O₄ where “M” represents the alkaline earth metal,such as Mg, Ca, Sr or Ba. Such compounds generally comprise a spinelstructure. These compounds can be prepared using conventional methodswell known in the art or, for example, by using a method described in EP0945165, U.S. Pat. Nos. 6,217,837 or 6,517,795.

Typically, the alkaline earth metal aluminate is magnesium aluminate(MgAl₂O₄), calcium aluminate (CaAl₂O₄), strontium aluminate (SrAl₂O₄),or barium aluminate (BaAl₂O₄), or a mixture of two or more thereof.Preferably, the alkaline earth metal aluminate is magnesium aluminate(MgAl₂O₄).

Generally, when the support material comprises an alkaline earth metalaluminate, then the alkaline earth metal (“M”) of the alkaline earthmetal aluminate is different to the alkaline earth metal component. Itis preferred that the alkaline earth metal aluminate and the alkalineearth metal component comprise different alkaline earth metals.

The oxidation catalyst of the invention generally comprises a totalamount of support material of 0.1 to 5 g in⁻³, preferably 0.2 to 4 gin⁻³ (e.g. 0.5 to 3.5 g in⁻³). When the oxidation catalyst comprises asecond support material, in addition to the support material comprisingthe modified alumina, then the total amount refers to the amount of boththe second support material and the support material comprising themodified alumina.

When the oxidation catalyst is used as a diesel oxidation catalyst, thengenerally the total amount of support material is 1 to 2.5 g in⁻³. Whenthe oxidation catalyst is used as a catalysed soot filter, then thetotal amount of support material is generally 0.2 to 4 g in⁻³.

When the oxidation catalyst comprises a second support material, thentypically the amount of the support material comprising the modifiedalumina is 0.1 to 3.0 g in⁻³, preferably 0.2 to 2.5 g in⁻³, still morepreferably 0.3 to 2.0, and even more preferably 0.5 to 1.75 g in⁻³.

In general, the ratio of the total mass of the alkaline earth metalcomponent to the total mass of the support material comprising themodified alumina is 1:200 to 1:5, preferably 1:150 to 1:10, even morepreferably 1:100 to 1:20.

Typically, the support material, particularly the alumina doped with aheteroatom component, is in particulate form. The support material mayhave a d₉₀ particle size of ≦20 μm (as determined by conventional laserdiffraction techniques). The particle size distribution of the supportmaterial is selected to aid adhesion to the substrate. The particles aregenerally obtained by milling.

Generally, the support material has a specific surface area of 50 to 500m² g⁻¹ (measured by BET in accordance with DIN 66131 or after activationat 550° C. for 3 hours). It is preferred that the support material has aspecific surface area of 50 to 300 m² g⁻¹, more preferably 100 to 250 m²g⁻¹.

The oxidation catalyst optionally further comprises a second supportmaterial. Typically, the alkaline earth metal component is disposed orsupported on the support material comprising the modified alumina and/ora second support material. When the oxidation catalyst comprises aplurality of layers, then the second support material and the supportmaterial comprising the modified alumina are preferably in differentlayers.

In general, the alkaline earth metal component is disposed or supportedon at least one support material that comprises, or consists essentiallyof, a modified alumina incorporating a heteroatom component. Typically,the catalyst of the invention comprises a single support material, whichsupport material comprises, or consists essentially of, the modifiedalumina incorporating a heteroatom component.

If a second support material is present, especially when the secondsupport material is in the same layer as the first support material,then it is preferred that the alkaline earth metal component issubstantially disposed or supported on the support material comprisingthe modified alumina (the term “substantially” in this context refers toat least 90%, preferably at least 99%, more preferably at least 99%, ofthe mass of the alkaline earth component that is present, typically inthe layer or otherwise, is disposed on the support material comprisingthe modified alumina). It is further preferred that the alkaline earthmetal component is only disposed or supported on the support materialcomprising the modified alumina. For some combinations of supportmaterials in the same layer, it may be difficult to control the preciselocation of the alkaline earth metal component because of its solubilityand the alkaline earth metal component may be disposed or support on allof the support materials.

The oxidation catalyst also comprises a platinum group metal (PGM)component selected from the group consisting of a platinum (Pt)component, a palladium (Pd) component and a combination thereof. Theoxidation catalyst of the invention may comprise a single platinum groupmetal (PGM) component, which is either a platinum (Pt) component or apalladium (Pd) component.

Generally, it is preferred that the oxidation catalyst comprises aplatinum (Pt) component and a palladium (Pd) component (i.e. theplatinum group metal (PGM) component is a platinum (Pt) component and apalladium (Pd) component). The ratio of the total mass of the platinum(Pt) component to the total mass of the palladium (Pd) component istypically 3:1 to 1:3, preferably 2:1 to 1:2, and more preferably 1.5:1to 1:1.5, especially when, but not exclusively, the oxidation catalystcomprises a plurality of layers.

Typically, the total amount of the platinum group metal (PGM) component(e.g. the total amount of the platinum (Pt) component and/or thepalladium (Pd) component) is 5 to 500 g ft⁻³. Preferably, the totalamount of the PGM component is 10 to 400 g ft⁻³, more preferably 20 to300 g ft⁻³, still more preferably, 25 to 250 g ft⁻³, and even morepreferably 35 to 200 g ft⁻³.

When the oxidation catalyst is used as a diesel oxidation catalyst, thengenerally the total amount of the platinum group metal (PGM) componentis 25 to 200 g ft⁻³, more preferably 40 to 160 g ft⁻³. When theoxidation catalyst is used as a catalysed soot filter, then the totalamount of the platinum group metal (PGM) component is 5 to 100 g ft⁻³,more preferably 10 to 40 g ft⁻³.

Typically, the oxidation catalyst comprises a total amount by mass ofthe platinum group metal (PGM) component of 2.0 to 8.0 g. The totalamount of PGM component that is used depends on, amongst other things,the size of the substrate and the intended application of the oxidationcatalyst.

In addition to the platinum group metal (PGM) component, the oxidationcatalyst of the invention may further comprise a noble metal component.The noble metal component comprises a noble metal selected from thegroup consisting of ruthenium (Ru), rhodium (Rh), iridium (Ir), gold(Au), silver (Ag) and a combination of two or more thereof. It ispreferred that the noble metal component comprises a noble metalselected from the group consisting of gold, silver and a combinationthereof. More preferably, the noble metal component comprises, orconsists of, gold. When the catalyst comprises gold (Au), then aplatinum group metal (PGM) component, preferably a palladium (Pd)component, is present as an alloy with gold (Au) (e.g. a palladium-goldalloy). Catalysts comprising gold (Au) can be prepared using the methoddescribed in WO 2012/120292 by the present Applicant.

The oxidation catalyst of the invention optionally further comprises ahydrocarbon adsorbent. The hydrocarbon adsorbent may be selected from azeolite, active charcoal, porous graphite and a combination of two ormore thereof. It is preferred that the hydrocarbon adsorbent is azeolite. More preferably, the zeolite is a medium pore zeolite (e.g. azeolite having a maximum ring size of eight tetrahedral atoms) or alarge pore zeolite (e.g. a zeolite having a maximum ring size of tentetrahedral atoms). Examples of suitable zeolites or types of zeoliteinclude faujasite, clinoptilolite, mordenite, silicalite, ferrierite,zeolite X, zeolite Y, ultrastable zeolite Y, AEI zeolite, ZSM-5 zeolite,ZSM-12 zeolite, ZSM-20 zeolite, ZSM-34 zeolite, CHA zeolite, SSZ-3zeolite, SAPO-5 zeolite, offretite, a beta zeolite or a copper CHAzeolite. The zeolite is preferably ZSM-5, a beta zeolite or a Y zeolite.

Typically, the zeolite has a silica to alumina molar ratio of at least25:1, preferably at least 25:1, with useful ranges of from 25:1 to1000:1, 50:1 to 500:1 as well as 25:1 to 100:1, 25:1 to 300:1, from100:1 to 250:1. Zeolites having a high molar ratio of silica to aluminashow improved hydrothermal stability.

When the catalyst comprises a hydrocarbon adsorbent, then typically thetotal amount of hydrocarbon adsorbent is 0.05 to 3.00 g in⁻³,particularly 0.10 to 2.00 g in⁻³, more particularly 0.2 to 0.8 g in⁻³.

The catalyst of the invention optionally further comprises an oxygenstorage material. Such materials are well-known in the art. The oxygenstorage material may be selected from ceria (CeO₂) and ceria-zirconia(CeO₂—ZrO₂), such as a ceria-zirconia solid solution.

Typically, at least one platinum group metal (PGM) component issupported on the support material comprising the modified aluminaincorporating a heteroatom component. Thus, a platinum (Pt) component ora palladium (Pd) component or both a platinum (Pt) component and apalladium (Pd) component is supported on the support material.

Generally, the alkaline earth metal component and at least one platinumgroup metal (PGM) component is supported on the support materialcomprising the modified alumina incorporating a heteroatom component.Thus, the oxidation catalyst of the invention may comprise a palladium(Pd) component and/or a platinum (Pt) component and an alkaline earthmetal component supported on the same support material, namely thesupport material comprising the modified alumina incorporating aheteroatom component. It is preferred that a palladium (Pd) component, aplatinum (Pt) component and an alkaline earth metal component aresupported on the support material comprising the modified aluminaincorporating a heteroatom component.

As mentioned above, the oxidation catalyst may or may not furthercomprise a second support material. The second support material may beselected from the group consisting of alumina, silica, alumina-silica,zirconia, titania, ceria and a mixture of two or more thereof. Thesecond support material is preferably selected from the group consistingof alumina, silica, zirconia, titania and a mixture of two or morethereof, particularly alumina, silica, titania and a mixture of two ormore thereof. More preferably, the second support material comprises, orconsists of, alumina.

When the oxidation catalyst comprises a second support material, thenpreferably at least one platinum group metal (PGM) component issupported on the second support material. A platinum (Pt) component, apalladium (Pd) component or both a platinum (Pt) component and apalladium (Pd) component may be supported on the second supportmaterial.

In addition or as alternative to being supported on the support materialcomprising the modified alumina, the alkaline earth metal component maybe supported on the second support material. However, it is preferredthat the alkaline earth metal component is only supported on the supportmaterial comprising the modified alumina (i.e. the alkaline earth metalcomponent is not supported on the second support material).

If the oxidation catalyst comprises a noble metal component and/or anoxygen storage material, then the noble metal component and/or theoxygen storage material may be supported on the support materialcomprising the modified alumina and/or, if present, the second supportmaterial. When the oxidation catalyst additionally comprises an oxygenstorage material and a second support material, then the oxygen storagematerial and the second support material are different (e.g. the oxygenstorage material and the second support material are not both ceria orceria-zirconia).

Generally, the platinum group metal (PGM) component(s), the alkalineearth metal component, the support material and any optional noble metalcomponent, oxygen storage material, hydrocarbon adsorbent and/or secondstorage material are disposed or supported on the substrate.

The oxidation catalyst of the invention comprises a substrate. Theoxidation catalyst may comprise a plurality of substrates (e.g. 2, 3 or4 substrates), more preferably two substrates (i.e. only twosubstrates). When there are two substrates, then a first substrate maybe in contact with or separate from a second substrate. When the firstsubstrate is separate from the second substrate, then preferably thedistance (e.g. the perpendicular distance between faces) between anoutlet end (e.g. the face at an outlet end) of the first substrate andinlet end (e.g. the face at an inlet end) of the second substrate isfrom 0.5 mm to 50 mm, preferably 1 mm to 40 mm, more preferably 1.5 mmto 30 mm (e.g. 1.75 mm to 25 mm), such as 2 mm to 20 mm (e.g. 3 mm to 15mm), and still more preferably 5 mm to 10 mm.

In general, it is preferred that the oxidation catalyst comprises asingle substrate (i.e. only one substrate).

Substrates for supporting oxidation catalysts for treating the exhaustgas of a compression ignition engine are well known in the art.Generally, the substrate is a ceramic material or a metallic material.

It is preferred that the substrate is made or composed of cordierite(SiO₂—Al₂O₃—MgO), silicon carbide (SiC), Fe—Cr—Al alloy, Ni—Cr—Al alloy,or a stainless steel alloy.

Typically, the substrate is a monolith. It is preferred that themonolith is a flow-through monolith or a filtering monolith.

In general, the oxidation catalyst of the invention is for use as adiesel oxidation catalyst (DOC) or a catalysed soot filter (CSF). Inpractice, catalyst formulations employed in DOCs and CSFs are similar.Generally, however, a principle difference between a DOC and a CSF isthe substrate onto which the catalyst formulation is coated and theamount of PGM component in the coating.

A flow-through monolith typically comprises a honeycomb monolith (e.g. ametal or ceramic honeycomb monolith) having a plurality of channelsextending therethrough, which channels are open at both ends. When thesubstrate is a flow-through monolith, then the oxidation catalyst of theinvention is typically a diesel oxidation catalyst (DOC) or is for useas a diesel oxidation catalyst (DOC).

A filtering monolith generally comprises a plurality of inlet channelsand a plurality of outlet channels, wherein the inlet channels are openat an upstream end (i.e. exhaust gas inlet side) and are plugged orsealed at a downstream end (i.e. exhaust gas outlet side), the outletchannels are plugged or sealed at an upstream end and are open at adownstream end, and wherein each inlet channel is separated from anoutlet channel by a porous structure. When the substrate is a filteringmonolith, then the oxidation catalyst of the invention is typically acatalysed soot filter (CSF) or is for use as a catalysed soot filter(CSF).

When the monolith is a filtering monolith, it is preferred that thefiltering monolith is a wall-flow filter. In a wall-flow filter, eachinlet channel is alternately separated from an outlet channel by a wallof the porous structure and vice versa. It is preferred that the inletchannel and the outlet channels have a honeycomb arrangement. When thereis a honeycomb arrangement, it is preferred that the channels verticallyand laterally adjacent to an inlet channel are plugged at an upstreamend and vice versa (i.e. the channels vertically and laterally adjacentto an outlet channel are plugged at a downstream end). When viewed fromeither end, the alternately plugged and open ends of the channels takeon the appearance of a chessboard.

In principle, the substrate may be of any shape or size. However, theshape and size of the substrate is usually selected to optimise exposureof the catalytically active materials in the catalyst to the exhaustgas. The substrate may, for example, have a tubular, fibrous orparticulate form. Examples of suitable supporting substrates include asubstrate of the monolithic honeycomb cordierite type, a substrate ofthe monolithic honeycomb SiC type, a substrate of the layered fibre orknitted fabric type, a substrate of the foam type, a substrate of thecrossflow type, a substrate of the metal wire mesh type, a substrate ofthe metal porous body type and a substrate of the ceramic particle type.

Generally, the oxidation catalyst of the invention comprises a singlelayer or a plurality of layers (e.g. 2, 3 or 4 layers) disposed on thesubstrate. Typically, each layer is formed by applying a washcoatcoating onto the substrate.

The oxidation catalyst of the invention may comprise, or consist of, asubstrate and a single layer disposed on the substrate, wherein thesingle layer comprises a platinum group metal (PGM) component selectedfrom the group consisting of a platinum (Pt) component, a palladium (Pd)component and a combination thereof; an alkaline earth metal component;and the support material comprising the modified alumina incorporating aheteroatom component. The single layer may further comprise a noblemetal component and/or an oxygen storage material and/or a hydrocarbonadsorbent and/or a second storage material. It is preferred that thesingle layer further comprises a hydrocarbon adsorbent and optionally anoxygen storage material.

When the oxidation catalyst comprises, or consists of, a substrate and asingle layer disposed on the substrate, then preferably the single layercomprises a platinum (Pt) component and a palladium (Pd) component (i.e.the platinum group metal (PGM) component is a platinum (Pt) componentand a palladium (Pd) component). When the single layer comprises aplatinum (Pt) component and a palladium (Pd) component, then therelative amount of the platinum (Pt) component to the palladium (Pd)component can vary.

Typically, the ratio by mass of the platinum (Pt) component to thepalladium (Pd) component is ≧35:65 (e.g. ≧7:13). It is preferred thatthe ratio by mass of the platinum (Pt) component to the palladium (Pd)component is ≧40:60 (e.g. ≧2:3), more preferably ≧42.5:57.5 (e.g.≧17:23), particularly ≧45:55 (e.g. ≧9:11), such as ≧47.5:52.5 (e.g.≧19:21), and still more preferably ≧50:50 (e.g. ≧1:1). The ratio by mass(i.e. mass ratio) of the platinum (Pt) component to the palladium (Pd)component is typically 80:20 to 35:65 (e.g. 4:1 to 7:13). It ispreferred that the ratio by mass of the platinum (Pt) component to thepalladium (Pd) component is 75:25 to 40:60 (e.g. 3:1 to 2:3), morepreferably 70:30 to 42.5:57.5 (e.g. 7:3 to 17:23), even more preferably67.5:32.5 to 45:55 (e.g. 27:13 to 9:11), such as 65:35 to 47.5:52.5(e.g. 13:7 to 19:21), and still more preferably 60:40 to 50:50 (e.g. 3:2to 1:1).

It is thought that oxidation catalysts where the mass of the palladium(Pd) component is less than the mass of the platinum (Pt) component haveadvantageous activity. Thus, the catalyst of the invention preferablycomprises the platinum (Pt) component and the palladium (Pd) componentin a ratio by mass of 65:35 to 52.5:47.5 (e.g. 13:7 to 21:19), morepreferably 60:40 to 55:45 (e.g. 3:2 to 11:9).

Typically, the ratio by mass (i.e. mass ratio) of the alkaline earthmetal component to the platinum group metal (PGM) component is 0.25:1 to20:1. It is preferred that the mass ratio of the alkaline earth metalcomponent to the platinum group metal (PGM) component is 0.5:1 to 17:1,more preferably 1:1 to 15:1, particularly 1.5:1 to 10:1, still morepreferably 2:1 to 7.5:1, and even more preferably 2.5:1 to 5:1.

The oxidation catalyst preferably comprises a plurality of layers, suchas 2, 3 or 4 layers.

When there is a plurality of layers, then the oxidation catalyst maycomprise a plurality of substrates, preferably two substrates. Whenthere is a plurality of substrates (e.g. two substrates), thenpreferably a first layer is disposed on a first substrate and a secondlayer is disposed on a second substrate. Thus, any reference below tothe first layer being disposed on the substrate may refer to the firstlayer being disposed on the first substrate. Similarly, any referencebelow to the second layer being disposed on the second substrate mayrefer to the second layer being disposed on the second substrate.

When there is a plurality of substrates, then the first substrate may beupstream of the second substrate. Alternatively, the second substratemay be upstream of the first substrate.

In general, it is preferred that the oxidation catalyst comprises asingle substrate, particularly when the oxidation catalyst comprises aplurality of layers.

When there is a plurality of layers, then generally a first layer isdisposed on the substrate (e.g. the first layer is preferably disposeddirectly on the substrate, such that the first layer is in contact witha surface of the substrate). The first layer may be disposed on a thirdlayer or a fourth layer. It is preferable that the first layer isdisposed directly on the substrate.

A second layer may be disposed on the substrate (e.g. to form a zone asdescribed below, which is separate from, or partly overlaps with, thefirst layer) or the second layer may be disposed on the first layer.

When the second layer is disposed on the first layer, it may completelyor partly overlap (i.e. cover) the first layer. If the catalystcomprises a third layer, then the third layer may be disposed on thesecond layer and/or the first layer, preferably the third layer isdisposed on the first layer. If the catalyst comprises a fourth layer,then the fourth layer may be disposed on the third layer and/or thesecond layer.

When the second layer is disposed on the substrate (e.g. to form azone), then the second layer may be disposed directly on the substrate(i.e. the second layer is in contact with a surface of the substrate) orit may be disposed on a third layer or a fourth layer.

The first layer may be a zone (e.g. a first zone) and/or the secondlayer may be a zone (e.g. a second zone). For the avoidance of doubt,features described herein relating to the “first layer” and “secondlayer”, especially the composition of the “first layer” and the “secondlayer”, also relate to the “first zone” and “second zone” respectively.

The first layer may be a first zone and the second layer may be a secondzone, such as when the first zone and second zone are side-by-side onthe same substrate or the first zone is disposed on a first substrateand a second zone is disposed on a second substrate (i.e. the firstsubstrate and the second substrate are different) and the firstsubstrate and second substrate are side-by-side. Preferably, the firstzone and second zone are disposed on the same substrate.

The first zone may be upstream of the second zone. When the first zoneis upstream of the second zone, inlet exhaust gas will contact the firstzone before the second zone. Alternatively, the second zone may beupstream of the first zone. Similarly, when the second zone is upstreamof the first zone, inlet exhaust gas will contact the second zone beforethe first zone.

When the first zone and the second zone are disposed on the samesubstrate, then the first zone may abut the second zone or the firstzone may be separate from the second zone. If the first zone abuts thesecond zone, then preferably the first zone is in contact with thesecond zone. When the first zone is separate from the second zone, thentypically there is a gap or space between the first zone and the secondzone.

Typically, the first zone has a length of 10 to 80% of the length of thesubstrate (e.g. 10 to 45%), preferably 15 to 75% of the length of thesubstrate (e.g. 15 to 40%), more preferably 20 to 60% (e.g. 25 to 45%)of the length of the substrate, still more preferably 25 to 50%.

The second zone typically has a length of 10 to 80% of the length of thesubstrate (e.g. 10 to 45%), preferably 15 to 75% of the length of thesubstrate (e.g. 15 to 40%), more preferably 20 to 60% (e.g. 25 to 45%)of the length of the substrate, still more preferably 25 to 50%.

A preferred oxidation catalyst comprises two layers (e.g. only twolayers), wherein a first layer is disposed on the substrate and a secondlayer is disposed on the first layer.

Typically, the second layer completely or partly overlaps the firstlayer.

The first layer and the second layer may have different lengths, or thefirst layer and the second layer may have about the same length.Generally, the length of the first layer and the length of the secondlayer is each substantially uniform.

The first layer typically extends for substantially an entire length ofthe channels in the substrate, particularly when the substrate is amonolith.

In an oxidation catalyst comprising a plurality of layers, the secondlayer may be arranged in a zone of substantially uniform length at adownstream end of the substrate. It is preferred that the zone at thedownstream end is nearer to the outlet end of the substrate than to theinlet end. Methods of making differential length layered coatings areknown in the art (see for example WO 99/47260 by the present Applicant).

When the oxidation catalyst comprises a plurality of layers, then theplatinum group metal (PGM) component, the alkaline earth metal componentand the support material comprising the modified alumina can bedistributed amongst the layers in a variety of ways.

In general, the first layer (or first zone) comprises a platinum groupmetal (PGM) component selected from the group consisting of a platinum(Pt) component, a palladium (Pd) component and a combination thereof,and the second layer (or second zone) comprises a platinum group metal(PGM) component selected from the group consisting of a platinum (Pt)component, a palladium (Pd) component and a combination thereof. It ispreferred that the first layer/zone is different (e.g. in composition)to the second layer/zone. For example, the first and second layers/zonesmay comprise different platinum group metal (PGM) components and/or thefirst and second layers/zones may comprise a different total amount ofthe platinum group metal (PGM) component.

In a first embodiment, the first layer (or first zone) comprises a PGMcomponent selected from the group consisting of a Pd component and acombination of (i.e. both) a Pd component and a Pt component, and thesecond layer (or second zone) comprises a PGM component consisting of aPt component. This means that the first layer/zone comprises a Pdcomponent and optionally a Pt component as the only PGM component, andthe second layer/zone comprises a Pt component as the only PGMcomponent. Preferably, the first layer/zone comprises a PGM componentconsisting of a combination of (i.e. both) a Pd component and a Ptcomponent. Thus, it is preferred that the first layer/zone comprisesboth a Pt component and a Pd component as the only PGM component, andthe second layer/zone comprises a Pt component as the only PGMcomponent.

Typically, in the first embodiment, the first layer (or first zone)further comprises the alkaline earth metal component and the supportmaterial comprising a modified alumina incorporating a heteroatomcomponent, and/or the second layer (or second zone) further comprisesthe alkaline earth metal component and the support material comprising amodified alumina incorporating a heteroatom component. It is preferredthat the first layer/zone further comprises the alkaline earth metalcomponent and the support material comprising a modified aluminaincorporating a heteroatom component.

When the first layer/zone comprises a Pd component as the only PGMcomponent, then the first layer/zone may comprise a second supportmaterial. Preferably the second support material is ceria,ceria-zirconia, alumina or silica-alumina. The second support materialmay be ceria. The second support material may be ceria-zirconia. Thesecond support material may be alumina. The second support material maybe silica-alumina. More preferably, the first layer/zone comprises a PGMcomponent selected from the group consisting of a Pd component, and asecond support material, wherein the second support material is ceria.

In a second embodiment, the first layer (or first zone) comprises a PGMcomponent selected from the group consisting of a Pt component and acombination of (i.e. both) a Pd component and a Pt component, and thesecond layer (or second zone) comprises a PGM component consisting of aPd component. This means that the first layer/zone comprises a Ptcomponent and optionally a Pd component as the only PGM component, andthe second layer/zone comprises a Pd component as the only PGMcomponent.

Preferably, the first layer/zone comprises a PGM component consisting ofa combination of (i.e. both) a Pd component and a Pt component. Thus, itis preferred that the first layer/zone comprises both a Pt component anda Pd component as the only PGM component, and the second layer/zonecomprises a Pd component as the only PGM component. Typically, theamount of the Pt component in the first layer/zone is greater than theamount of the Pd component in the first layer/zone (the amount beingmeasured in g ft⁻³ or as a molar amount).

In the second embodiment, the first layer (or first zone) may furthercomprise the alkaline earth metal component and the support materialcomprising a modified alumina incorporating a heteroatom component,and/or the second layer (or second zone) may further comprise thealkaline earth metal component and the support material comprising amodified alumina incorporating a heteroatom component. It is preferredthat the first layer/zone further comprises the alkaline earth metalcomponent and the support material comprising a modified aluminaincorporating a heteroatom component.

In the second embodiment, the second layer/zone typically comprises asecond support material. Preferably the second support material isceria, ceria-zirconia, alumina or silica-alumina. The second supportmaterial may be ceria. The second support material may beceria-zirconia. The second support material may be alumina. The secondsupport material may be silica-alumina.

In a third embodiment, the first layer (or first zone) comprises a PGMcomponent selected from the group consisting of a Pt component and a Pdcomponent, and the second layer (or second zone) comprises a PGMcomponent consisting of a combination of (i.e. both) a Pd component anda Pt component. This means that the first layer/zone comprises a Ptcomponent or a Pd component as the only PGM component, and the secondlayer/zone comprises a Pt component and a Pd component as the only PGMcomponent. Preferably, the first layer/zone comprises a PGM componentconsisting of a Pt component. Thus, it is preferred that the firstlayer/zone comprises a Pt component as the only PGM component, and thesecond layer/zone comprises a Pt component and a Pd component as theonly PGM component.

In the third embodiment, when the first layer/zone comprises a Ptcomponent as the only PGM component, then typically the ratio by mass ofthe Pt component in the second layer/zone to the Pd component in thesecond layer/zone is ≦2:1, preferably <2:1. When the first layer/zonecomprises a Pd component as the only PGM component, then typically theamount of the Pd component in the second layer/zone is less than theamount of the Pt component in the second layer/zone (the amount beingmeasured in g ft⁻³ or is a molar amount).

Typically, in the third embodiment, the first layer (or first zone)further comprises the alkaline earth metal component and the supportmaterial comprising a modified alumina incorporating a heteroatomcomponent, and/or the second layer (or second zone) further comprisesthe alkaline earth metal component and the support material comprising amodified alumina incorporating a heteroatom component. When the firstlayer/zone comprises a Pt component as the only PGM component, then itis preferred that the first layer/zone further comprises the alkalineearth metal component and the support material comprising a modifiedalumina incorporating a heteroatom component. When the first layer/zonecomprises a Pd component as the only PGM component, then it is preferredthat the second layer/zone further comprises the alkaline earth metalcomponent and the support material comprising a modified aluminaincorporating a heteroatom component.

In the third embodiment, when the first layer/zone comprises a Pdcomponent as the only PGM component, then the first layer/zone maycomprise a second support material. Preferably the second supportmaterial is ceria, ceria-zirconia, alumina or silica-alumina. The secondsupport material may be ceria. The second support material may beceria-zirconia. The second support material may be alumina. The secondsupport material may be silica-alumina.

In a fourth embodiment, the first layer (or first zone) comprises a PGMcomponent consisting of a combination of (i.e. both) a Pt component anda Pd component, and the second layer (or second zone) comprises a PGMcomponent consisting of a combination of (i.e. both) a Pd component anda Pt component. This means that first layer/zone comprises a Ptcomponent and a Pd component as the only PGM component, and the secondlayer/zone comprises a Pt component or a Pd component as the only PGMcomponent. In the fourth embodiment, the first layer/zone and secondlayer/zone typically comprise a different ratio by mass of the Ptcomponent to the Pd component. Thus, the ratio by mass of the Ptcomponent to the Pd component in the first layer/zone is different tothe ratio by mass of the Pt component to the Pd component in thesecond/zone layer.

In the fourth embodiment, when the amount of the Pd component in thefirst layer/zone is less than the amount of the Pt component in thefirst layer/zone (the amount being measured in gft⁻³ or is a molaramount), then preferably the amount of the Pd component in the secondlayer/zone is greater than the amount of the Pt component in the secondlayer/zone. Alternatively, when the amount of the Pd component in thefirst layer/zone is greater than the amount of the Pt component in thefirst layer/zone (the amount being measured in g ft⁻³ or is a molaramount), then preferably the amount of the Pd component in the secondlayer/zone is less than the amount of the Pt component in the secondlayer/zone.

Generally, the ratio by mass of the platinum (Pt) component to thepalladium (Pd) component, particularly in first layer/zone of the firstor second embodiments, the second layer/zone of the third embodiment, orthe first layer/zone and/or second layer/zone of the fourth embodiment,preferably the second layer/zone of the fourth embodiment, is ≧35:65(e.g. ≧7:13). It is preferred that the ratio by mass of the platinum(Pt) component to the palladium (Pd) component is ≧40:60 (e.g. ≧2:3),more preferably ≧42.5:57.5 (e.g. ≧17:23), particularly ≧45:55 (e.g.≧9:11), such as ≧47.5:52.5 (e.g. ≧19:21), and still more preferably≧50:50 (e.g. ≧1:1).

It is preferred that the ratio by mass of the platinum (Pt) component tothe palladium (Pd) component, particularly in first layer/zone of thefirst or second embodiments, the second layer/zone of the thirdembodiment, or the first layer/zone and/or second layer/zone of thefourth embodiment, preferably the second layer/zone of the fourthembodiment, is 80:20 to 35:65 (e.g. 4:1 to 7:13), particularly 75:25 to40:60 (e.g. 3:1 to 2:3), more preferably 70:30 to 42.5:57.5 (e.g. 7:3 to17:23), even more preferably 67.5:32.5 to 45:55 (e.g. 27:13 to 9:11),such as 65:35 to 47.5:52.5 (e.g. 13:7 to 19:21), and still morepreferably 60:40 to 50:50 (e.g. 3:2 to 1:1). For the second layer of thethird embodiment, it is particularly preferable that the ratio by massof the platinum (Pt) component to the palladium (Pd) component is 2:1 to7:13, particularly 13:7 to 2:3, more preferably 60:40 to 50:50 (e.g. 3:2to 1:1)

It is thought that oxidation catalysts where the mass of the palladium(Pd) component is less than the mass of the platinum (Pt) component haveadvantageous activity, especially when both a platinum (Pt) component, apalladium (Pd) component and an alkaline earth metal component arepresent in the same layer/zone. Thus, in the first layer/zone of thefirst embodiment, the first layer/zone of the second embodiment, thesecond layer/zone of the third embodiment, or the first layer/zoneand/or second layer/zone of the fourth embodiment, preferably the secondlayer/zone of the fourth embodiment, the oxidation catalyst of theinvention preferably comprises the platinum (Pt) component and thepalladium (Pd) component in a ratio by mass of 65:35 to 52.5:47.5 (e.g.13:7 to 21:19), more preferably 60:40 to 55:45 (e.g. 3:2 to 11:9).

In a fifth embodiment, the first layer (or first zone) comprises a PGMcomponent selected from the group consisting of a Pt component and a Pdcomponent, and the second layer (or second zone) comprises a PGMcomponent selected from the group consisting of a Pd component and a Ptcomponent, and wherein the first and second layer/zone each comprise thesame PGM component. This means that the first layer/zone and the secondlayer/zone each comprise a Pt component or a Pd component as the onlyPGM component. Typically, the total amount of PGM component in the firstlayer/zone is different to the total amount of PGM component in thesecond layer/zone.

When both the first layer/zone and the second layer/zone each comprise aPd component as the only PGM component, then preferably the firstlayer/zone comprises a second support material and/or the secondlayer/zone comprises a second support material. It is preferred that thesecond support material is ceria, ceria-zirconia, alumina orsilica-alumina. The second support material may be ceria. The secondsupport material may be ceria-zirconia. The second support material maybe alumina. The second support material may be silica-alumina.

In the first to fifth embodiments, the first layer/zone may comprise analkaline earth metal component and/or the second layer/zone may comprisean alkaline earth metal component. When the first layer/zone comprisesthe alkaline earth metal component, the second layer/zone may notcomprise an alkaline earth metal component. Alternatively, when thesecond layer/zone comprises the alkaline earth metal component, thefirst layer/zone may not comprise an alkaline earth metal component.

In the first to fifth embodiments, the first layer/zone may comprise thesupport material comprising the modified alumina, and/or the secondlayer/zone may comprise the support material comprising the modifiedalumina. Typically, it is preferred that a layer or zone comprising aplatinum (Pt) component also comprises the support material comprisingthe modified alumina.

In the first to fifth embodiments, the first layer/zone may comprise asecond support material and/or the second layer/zone may comprise asecond support material. The first layer/zone and the second layer/zonemay comprise different support materials. It is preferred that thesecond support material and the support material comprising the modifiedalumina are in different layers/zones.

In general, the alkaline earth metal component and the support materialcomprising the modified alumina are present in at least one of the samelayers/zones.

When the first layer/zone comprises the alkaline earth metal component,then typically the ratio of the mass of the alkaline earth metalcomponent to the mass of the platinum group metal (PGM) component in thefirst layer is 0.25:1 to 20:1, preferably 0.5:1 to 17:1, more preferably1:1 to 15:1, particularly 1.5:1 to 10:1, still more preferably 2:1 to7.5:1, and even more preferably 2.5:1 to 5:1.

When the second layer/zone comprises the alkaline earth metal component,then typically the ratio of the mass of the alkaline earth metalcomponent to the mass of the platinum group metal (PGM) component in thesecond layer is 0.25:1 to 20:1, preferably 0.5:1 to 17:1, morepreferably 1:1 to 15:1, particularly 1.5:1 to 10:1, still morepreferably 2:1 to 7.5:1, and even more preferably 2.5:1 to 5:1.

In the first to fifth embodiments, the first layer/zone may optionallyfurther comprise a noble metal component and/or an oxygen storagematerial and/or a hydrocarbon adsorbent. Preferably, the firstlayer/zone further comprises a hydrocarbon adsorbent.

In the first to fifth embodiments, the second layer/zone may optionallyfurther comprise a noble metal component and/or an oxygen storagematerial and/or a hydrocarbon adsorbent. Preferably, the secondlayer/zone further comprises a hydrocarbon adsorbent.

In one aspect of the first embodiment, the first layer/zone typicallycomprises a Pd component, a Pt component, an alkaline earth metalcomponent and the support material comprising the modified alumina; andthe second layer/zone comprises a Pt component and either a secondsupport material or a support material comprising the modified alumina,and optionally an alkaline earth metal component. When the secondlayer/zone comprises a second support material, then preferably thesecond support material is alumina.

In one aspect of the fourth embodiment, the first layer/zone comprises aPt component, a Pd component, an alkaline earth metal component and thesupport material comprising the modified alumina; and the secondlayer/zone comprises a Pt component, a Pd component and either a secondsupport material or a support material comprising the modified alumina,and optionally an alkaline earth metal component. It is preferred thatthe ratio by mass of the Pt component in the second layer/zone to the Pdcomponent in the second layer is ≦10:1 (e.g. 10:1 to 1:2), morepreferably ≦15:2 (e.g. 7.5:1 to 1:1.5), and still more preferably ≦5:1(e.g. 5:1 to 1.5:1). When the second layer/zone comprises a secondsupport material, then preferably the second support material isalumina.

When the first layer is a first zone and the second layer is a secondzone, then (a) in the first and third embodiments it is preferred thatthe first layer/zone is upstream of the second layer/zone, (b) in thesecond embodiment it is preferred that the second layer/zone is upstreamof the first layer/zone, and (c) in the fifth embodiment it is preferredthat the layer/zone comprising the second support material is upstreamof the layer/zone comprising the support material comprising themodified alumina.

In embodiments where there is a second support material, especially whenthe second support material is either ceria or ceria-zirconia, then itmay be advantageous to arrange the layer or zone comprising the secondsupport material to contact the exhaust gas after the other layer orzone. Thus, when there is a second support material, especially when thesecond support material is ceria or ceria-zirconia, it is preferred that(a) in the first and third embodiments it is preferred that the firstlayer/zone is downstream of the second layer/zone, (b) in the secondembodiment it is preferred that the second layer/zone is downstream ofthe first layer/zone, and (c) in the fifth embodiment it is preferredthat the layer/zone comprising the second support material is downstreamof the layer/zone comprising the support material comprising themodified alumina.

In general, the oxidation catalyst of the invention may or may notcomprise rhodium. It is preferred that the oxidation catalyst does notcomprise ruthenium, rhodium, and iridium.

Another general feature of the oxidation catalyst of the invention isthat when any cerium or ceria is present, then typically only theheteroatom component of the support material comprises the cerium orceria. It is further preferred that the oxidation catalyst of theinvention does not comprise ceria, particularly as a support material oras an oxygen storage material.

A further general feature of the oxidation catalyst of the invention isthat when an alkali metal, particularly sodium or potassium, andespecially potassium, is present, then preferably only the hydrocarbonadsorbent comprises the alkali metal, especially when the hydrocarbonadsorbent is a zeolite. It is further preferred that the oxidationcatalyst of the invention does not comprise an alkali metal,particularly sodium or potassium.

Another general feature of the invention is that the oxidation catalystof the invention does not comprise a NO_(x) adsorber composition. Thus,it is preferred that the oxidation catalyst of the invention is not aNO_(x) adsorber catalyst (also known as a NO_(x) trap) or is not for useas a NO_(x) adsorber catalyst.

Typically, the oxidation catalyst is for use as a diesel oxidationcatalyst (DOC) or a catalysed soot filter (CSF).

The first aspect of the invention relates to an exhaust system for acompression ignition engine, such as a diesel engine, which systemcomprises the oxidation catalyst defined above. The third aspect of theinvention relates to the use of the oxidation catalyst. The advantageousactivity of the oxidation catalyst of the invention, particularly itslow CO “light off” temperature, render it particularly suited for use incombination with certain other emissions control devices.

Typically, the exhaust system may further comprise, or the oxidationcatalyst is for use in combination with, at least one emissions controldevice. The emissions control device may be selected from a dieselparticulate filter (DPF), a NO_(x) adsorber catalyst (NAC), a leanNO_(x) catalyst (LNC), a selective catalytic reduction (SCR) catalyst, adiesel oxidation catalyst (DOC), a catalysed soot filter (CSF), aselective catalytic reduction filter (SCRF) catalyst, and combinationsof two or more thereof. Emissions control devices represented by theterms diesel particulate filters (DPFs), NO_(x) adsorber catalysts(NACs), lean NO_(x) catalysts (LNCs), selective catalytic reduction(SCR) catalysts, diesel oxidation catalyst (DOCs), catalysed sootfilters (CSFs) and selective catalytic reduction filter (SCRF) catalystsare all well known in the art.

Examples of emissions control devices for use with the oxidationcatalyst of the invention or for inclusion in the exhaust system of theinvention are provided below.

The diesel particulate filter preferably comprises a substrate, whereinthe substrate is a filtering monolith as defined above. The substratemay be coated with a catalyst formulation.

The catalyst formulation of the diesel particulate file may be suitablefor oxidising (i) particulate matter (PM) and/or (ii) carbon monoxide(CO) and hydrocarbons (HCs). When the catalyst formulation is suitablefor oxidising PM, then the resulting emissions control device is knownas a catalysed soot filter (CSF). Typically, the catalyst formulationcomprises a noble metal as defined above and/or platinum and/orpalladium.

The catalyst formulation of the diesel particulate filter may be aNO_(x) adsorber composition. When the catalyst formulation is a NO_(x)adsorber composition, the emissions control device is an example of aNO_(x) adsorber catalyst (NAC). Emissions control devices where thecatalyst formulation is a NO_(x) adsorber composition have beendescribed (see, for example, EP 0766993). NO_(x) adsorber compositionsare well known in the art (see, for example, EP 0766993 and U.S. Pat.No. 5,473,887). NO_(x) adsorber compositions are designed to adsorbNO_(x) from lean exhaust gas (lambda>1) and to desorb the NO when theoxygen concentration in the exhaust gas is decreased. Desorbed NO_(x)may then be reduced to N₂ with a suitable reductant (e.g. engine fuel)and promoted by a catalyst component, such as rhodium, of the NO_(x)adsorber composition itself or located downstream of the NO_(x) adsorbercomposition.

Modern NO_(x) absorber catalysts coated on honeycomb flow-throughmonolith substrates are typically arranged in layered arrangements.However, multiple layers applied on a filter substrate can createbackpressure problems. It is highly preferable, therefore, if the NO_(x)absorber catalyst for use in the present invention is a “single layer”NO_(x) absorber catalyst. Particularly preferred “single layer” NO_(x)absorber catalysts comprise a first component of rhodium supported on aceria-zirconia mixed oxide or an optionally stabilised alumina (e.g.stabilised with silica or lanthana or another rare earth element) incombination with second components which support platinum and/orpalladium. The second components comprise platinum and/or palladiumsupported on an alumina-based high surface area support and aparticulate “bulk” ceria (CeO₂) component, i.e. not a soluble ceriasupported on a particulate support, but “bulk” ceria capable ofsupporting the Pt and/or Pd as such. The particulate ceria comprises aNO_(x) absorber component and supports an alkaline earth metal and/or analkali metal, preferably barium, in addition to the platinum and/orpalladium. The alumina-based high surface area support can be magnesiumaluminate e.g. MgAl₂O₄, for example.

The preferred “single layer” NAC composition comprises a mixture of therhodium and platinum and/or palladium support components. Thesecomponents can be prepared separately, i.e. pre-formed prior tocombining them in a mixture, or rhodium, platinum and palladium saltsand the supports and other components can be combined and the rhodium,platinum and palladium components hydrolysed preferentially to depositonto the desired support.

Generally, a NO_(x) adsorber composition comprises an alkali metalcomponent, an alkaline earth metal component or a rare earth metalcomponent or a combination of two or more components thereof, whereinthe rare earth metal component comprises lanthanum or yttrium. It ispreferred that the alkali metal component comprises potassium or sodium,more preferably potassium. It is preferred that the alkaline earth metalcomponent comprises barium or strontium, more preferably barium.

The NO_(x) adsorber composition may further comprise a support materialand/or a catalytic metal component. The support material may be selectedfrom alumina, ceria, titania, zirconia and mixtures thereof. Thecatalytic metal component may comprise a metal selected from platinum(Pt), palladium (Pd), rhodium (Rh) and combinations of two or morethereof.

Lean NO_(x) catalysts (LNCs) are well known in the art. Preferred leanNO_(x) catalysts (LNC) comprises either (a) platinum (Pt) supported onalumina or (b) a copper exchanged zeolite, particularly copper exchangedZSM-5.

SCR catalysts are also well known in the art. When the exhaust system ofthe invention comprises an SCR catalyst, then the exhaust system mayfurther comprise an injector for injecting a nitrogenous reductant, suchas ammonia, or an ammonia precursor, such as urea or ammonium formate,preferably urea, into exhaust gas downstream of the catalyst foroxidising carbon monoxide (CO) and hydrocarbons (HCs) and upstream ofthe SCR catalyst. Such injector is fluidly linked to a source of suchnitrogenous reductant precursor, e.g. a tank thereof, andvalve-controlled dosing of the precursor into the exhaust stream isregulated by suitably programmed engine management means and closed loopor open loop feedback provided by sensors monitoring relevant exhaustgas composition. Ammonia can also be generated by heating ammoniumcarbamate (a solid) and the ammonia generated can be injected into theexhaust gas.

Alternatively or in addition to the injector, ammonia can be generatedin situ e.g. during rich regeneration of a NAC disposed upstream of thefilter or by contacting a DOC disposed upstream of the filter withengine-derived rich exhaust gas. Thus, the exhaust system may furthercomprise an engine management means for enriching the exhaust gas withhydrocarbons.

SCR catalysts for use in the present invention promote the reactionsselectively 4NH₃+4NO+O₂→4N₂+6H₂O (i.e. 1:1 NH₃:NO);4NH₃+2NO+2NO₂→4N₂+6H₂O (i.e. 1:1 NH₃:NO_(x); and 8NH₃+6NO₂→7N₂+12H₂O(i.e. 4:3 NH₃:NO_(x)) in preference to undesirable, non-selectiveside-reactions such as 2NH₃+2NO₂→N₂O+3H₂O+N₂.

The SCR catalyst may comprise a metal selected from the group consistingof at least one of Cu, Hf, La, Au, In, V, lanthanides and Group VIIItransition metals, such as Fe, which metal is supported on a refractoryoxide or molecular sieve. Particularly preferred metals are Ce, Fe andCu and combinations of any two or more thereof.

The refractory oxide may be selected from the group consisting of Al₂O₃,TiO₂, CeO₂, SiO₂, ZrO₂ and mixed oxides containing two or more thereof.The non-zeolite catalyst can also include tungsten oxide, e.g.V₂O₅/WO₃/TiO₂, WO_(x)/CeZrO₂, WO_(x)/ZrO₂ or Fe/WO_(x)/ZrO₂.

It is particularly preferred when an SCR catalyst or washcoat thereofcomprises at least one molecular sieve, such as an aluminosilicatezeolite or a SAPO. The at least one molecular sieve can be a small, amedium or a large pore molecular sieve, for example. By “small poremolecular sieve” herein we mean molecular sieves containing a maximumring size of 8, such as CHA; by “medium pore molecular sieve” herein wemean a molecular sieve containing a maximum ring size of 10, such asZSM-5; and by “large pore molecular sieve” herein we mean a molecularsieve having a maximum ring size of 12, such as beta. Small poremolecular sieves are potentially advantageous for use in SCRcatalysts—see for example WO 2008/132452.

Preferred molecular sieves with application as SCR catalysts in thepresent invention are synthetic aluminosilicate zeolite molecular sievesselected from the group consisting of AEI, ZSM-5, ZSM-20, ERI includingZSM-34, mordenite, ferrierite, BEA including Beta, Y, CHA, LEV includingNu-3, MCM-22 and EU-1, preferably AEI or CHA, and having asilica-to-alumina ratio of about 10 to about 50, such as about 15 toabout 40.

At its most basic, an ammonia slip catalyst (ASC) can be an oxidationcatalyst for oxidising ammonia which slips past an upstream SCR or SCRFcatalyst unreacted. The desired reaction (simplified) can be representedby 4NO+4NH₃+O₂→4N₂+6H₂O. Ammonia is a strong smelling compound andpotential irritant to animal mucosal surfaces, e.g. eyes and respiratorypathways, and so its emission to atmosphere should be limited so far aspossible. Possible ammonia slip catalysts include relatively low loadedplatinum group metals, preferably including Pt e.g. 1-15 g/ft³, on asuitable relatively high surface area oxide support, e.g. alumina coatedon a suitable substrate monolith.

In a particularly preferred arrangement, however, the platinum groupmetal and the support material (e.g. comprising a modified aluminaincorporating a heteroatom component) is disposed on a substrate (i.e. asubstrate monolith) in a first layer below an upper, second layeroverlying the first layer. The second layer is a SCR catalyst, selectedfrom any of those mentioned hereinabove, particularly molecular sievescontaining transition metals, such as Cu or Fe. A particularly preferredASC in the layered arrangement comprises CuCHA in the second or upperlayer.

In a first exhaust system embodiment, the exhaust system comprises theoxidation catalyst of the invention, preferably as a DOC, and acatalysed soot filter (CSF). Such an arrangement may be called aDOC/CSF. This embodiment also relates to the use of the oxidationcatalyst for treating an exhaust gas from a compression ignition enginein combination with a catalysed soot filter, preferably wherein theoxidation catalyst is, or is for use as, a diesel oxidation catalyst.The oxidation catalyst is typically followed by (e.g. is upstream of)the catalysed soot filter (CSF). Thus, for example, an outlet of theoxidation catalyst is connected to an inlet of the catalysed sootfilter.

The first exhaust system embodiment may further comprise a NO_(x)adsorber catalyst (NAC). Thus, the embodiment further relates to the useof the oxidation catalyst for treating an exhaust gas from a compressionignition engine in combination with a NO_(x) adsorber catalyst (NAC) anda catalysed soot filter (CSF), preferably wherein the oxidation catalystis, or is for use as, a diesel oxidation catalyst. Typically theoxidation catalyst is followed by (e.g. is upstream of) the NO_(x)adsorber catalyst (NAC), and the NO_(x) adsorber catalyst (NAC) isfollowed by (e.g. is upstream of) the catalysed soot filter (CSF).Generally, the oxidation catalyst, the NO_(x) adsorber catalyst (NAC)and the catalysed soot filter (CSF) are connected in series. Thus, forexample, an outlet of the oxidation catalyst is connected to an inlet ofthe NO_(x) adsorber catalyst (NAC), and an outlet of the NO adsorbercatalyst (NAC) is connected to an inlet of the catalysed soot filter(CSF). Such an arrangement may be termed a DOC/NAC/CSF.

In a second exhaust system embodiment, the exhaust system comprises adiesel oxidation catalyst and the oxidation catalyst of the invention,preferably as a catalysed soot filter (CSF). This arrangement may alsobe called a DOC/CSF arrangement. The embodiment further relates to theuse of the oxidation catalyst for treating an exhaust gas from acompression ignition engine in combination with a diesel oxidationcatalyst (DOC), preferably wherein the oxidation catalyst is, or is foruse as, a catalysed soot filter. Typically, the diesel oxidationcatalyst (DOC) is followed by (e.g. is upstream of) the oxidationcatalyst of the invention. Thus, an outlet of the diesel oxidationcatalyst is connected to an inlet of the oxidation catalyst of theinvention.

A third exhaust system embodiment relates to an exhaust systemcomprising the oxidation catalyst of the invention, preferably as a DOC,a catalysed soot filter (CSF) and a selective catalytic reduction (SCR)catalyst. Such an arrangement may be called a DOC/CSF/SCR and is apreferred exhaust system for a light-duty diesel vehicle. Thisembodiment also relates to the use of the oxidation catalyst fortreating an exhaust gas from a compression ignition engine incombination with a catalysed soot filter (CSF) and a selective catalyticreduction (SCR) catalyst, preferably wherein the oxidation catalyst is,or is for use as, a diesel oxidation catalyst. The oxidation catalyst istypically followed by (e.g. is upstream of) the catalysed soot filter(CSF). The catalysed soot filter is typically followed by (e.g. isupstream of) the selective catalytic reduction (SCR) catalyst. Anitrogenous reductant injector may be arranged between the catalysedsoot filter (CSF) and the selective catalytic reduction (SCR) catalyst.Thus, the catalysed soot filter (CSF) may be followed by (e.g. isupstream of) a nitrogenous reductant injector, and the nitrogenousreductant injector may be followed by (e.g. is upstream of) theselective catalytic reduction (SCR) catalyst.

A fourth exhaust system embodiment relates to an exhaust systemcomprising a diesel oxidation catalyst (DOC), the oxidation catalyst ofthe invention, preferably as a catalysed soot filter (CSF), and aselective catalytic reduction (SCR) catalyst. This is also a DOC/CSF/SCRarrangement. A further aspect of this embodiment relates to the use ofthe oxidation catalyst for treating an exhaust gas from a compressionignition engine in combination with a diesel oxidation catalyst (DOC)and a selective catalytic reduction (SCR) catalyst, preferably whereinthe oxidation catalyst is, or is for use as, a catalysed soot filter(CSF). The diesel oxidation catalyst (DOC) is typically followed by(e.g. is upstream of) the oxidation catalyst of the invention. Theoxidation catalyst of the invention is typically followed by (e.g. isupstream of) the selective catalytic reduction (SCR) catalyst. Anitrogenous reductant injector may be arranged between the oxidationcatalyst and the selective catalytic reduction (SCR) catalyst. Thus, theoxidation catalyst may be followed by (e.g. is upstream of) anitrogenous reductant injector, and the nitrogenous reductant injectormay be followed by (e.g. is upstream of) the selective catalyticreduction (SCR) catalyst.

In a fifth exhaust system embodiment, the exhaust system comprises theoxidation catalyst of the invention, preferably as a DOC, a selectivecatalytic reduction (SCR) catalyst and either a catalysed soot filter(CSF) or a diesel particulate filter (DPF). The arrangement is either aDOC/SCR/CSF or a DOC/SCR/DPF. This embodiment also relates to the use ofthe oxidation catalyst for treating an exhaust gas from a compressionignition engine in combination with a selective catalytic reduction(SCR) catalyst and either a catalysed soot filter (CSF) or a dieselparticulate filter (DPF), preferably wherein the oxidation catalyst is,or is for use as, a diesel oxidation catalyst.

In the fifth exhaust system embodiment, the oxidation catalyst of theinvention is typically followed by (e.g. is upstream of) the selectivecatalytic reduction (SCR) catalyst. A nitrogenous reductant injector maybe arranged between the oxidation catalyst and the selective catalyticreduction (SCR) catalyst. Thus, the oxidation catalyst may be followedby (e.g. is upstream of) a nitrogenous reductant injector, and thenitrogenous reductant injector may be followed by (e.g. is upstream of)the selective catalytic reduction (SCR) catalyst. The selectivecatalytic reduction (SCR) catalyst are followed by (e.g. are upstreamof) the catalysed soot filter (CSF) or the diesel particulate filter(DPF).

A sixth exhaust system embodiment comprises the oxidation catalyst ofthe invention, preferably as a DOC, and a selective catalytic reductionfilter (SCRF) catalyst. Such an arrangement may be called a DOC/SCRF.This embodiment also relates to the use of the oxidation catalyst fortreating an exhaust gas from a compression ignition engine incombination with a selective catalytic reduction filter (SCRF) catalyst,preferably wherein the oxidation catalyst is, or is for use as, a dieseloxidation catalyst. The oxidation catalyst of the invention is typicallyfollowed by (e.g. is upstream of) the selective catalytic reductionfilter (SCRF) catalyst. A nitrogenous reductant injector may be arrangedbetween the oxidation catalyst and the selective catalytic reductionfilter (SCRF) catalyst. Thus, the oxidation catalyst may be followed by(e.g. is upstream of) a nitrogenous reductant injector, and thenitrogenous reductant injector may be followed by (e.g. is upstream of)the selective catalytic reduction filter (SCRF) catalyst.

In a seventh exhaust system embodiment, the exhaust system comprises aNO_(x) adsorber catalyst (NAC) and the oxidation catalyst of theinvention, preferably as a catalysed soot filter (CSF). This arrangementmay also be called a NAC/CSF arrangement. The embodiment further relatesto the use of the oxidation catalyst for treating an exhaust gas from acompression ignition engine in combination with a NO_(x) adsorbercatalyst (NAC), preferably wherein the oxidation catalyst is, or is foruse as, a catalysed soot filter. Typically, the catalysed soot filter(CSF) is downstream of the NO_(x) adsorber catalyst (NAC). Thus, anoutlet of the NO_(x) adsorber catalyst (NAC) is connected to an inlet ofthe oxidation catalyst of the invention.

The seventh exhaust system embodiment may further comprise a selectivecatalytic reduction (SCR) catalyst. Thus, the embodiment further relatesto the use of the oxidation catalyst for treating an exhaust gas from acompression ignition engine in combination with a NO_(x) adsorbercatalyst (NAC) and a selective catalytic reduction (SCR) catalyst,preferably wherein the oxidation catalyst is, or is for use as, acatalysed soot filter (CSF). Typically the NO_(x) adsorber catalyst(NAC) is followed by (e.g. is upstream of) the oxidation catalyst of theinvention, and the oxidation catalyst of the invention is followed by(e.g. is upstream of) the selective catalytic reduction (SCR) catalyst.Such an arrangement may be termed a NAC/CSF/SCR. A nitrogenous reductantinjector may be arranged between the oxidation catalyst and theselective catalytic reduction (SCR) catalyst. Thus, the oxidationcatalyst may be followed by (e.g. is upstream of) a nitrogenousreductant injector, and the nitrogenous reductant injector may befollowed by (e.g. is upstream of) the selective catalytic reduction(SCR) catalyst. Alternatively, or additionally to the nitrogenousreductant injector, ammonia can be generated in situ e.g. during richregeneration of a NAC disposed upstream of the filter.

In the seventh exhaust system embodiment with an SCR catalyst, theNO_(x) adsorber catalyst (NAC), the oxidation catalyst and the selectivecatalytic reduction (SCR) catalyst are generally connected in serieswith an optional nitrogenous reductant injector being connected betweenthe oxidation catalyst and the selective catalytic reduction (SCR)catalyst. Thus, for example, an outlet of the NO_(x) adsorber catalyst(NAC) is connected to an inlet of the oxidation catalyst, and outlet ofthe oxidation catalyst is connected to an inlet of the selectivecatalytic reduction (SCR) catalyst.

In any of the first through seventh inclusive exhaust system embodimentsdescribed hereinabove containing a SCR catalyst (including SCRFcatalyst), an ASC catalyst can be disposed downstream from the SCRcatalyst or the SCRF catalyst (i.e. as a separate substrate monolith),or more preferably a zone on a downstream or trailing end of thesubstrate monolith comprising the SCR catalyst can be used as a supportfor the ASC.

A second aspect of the invention relates to a vehicle comprising acompression ignition engine and the exhaust system for the compressionignition engine.

The invention also provides a compression ignition engine comprising anexhaust system of the invention.

The compression ignition engine can be a homogenous charge compressionignition (HCCI) engine or a premixed charge compression ignition engine(PCCI) (see DieselNet Technology Guide “Engine Design for LowEmissions”, Revision 2010.12a) or more conventional Port Fuelinjected-type compression ignition engines.

Generally, the compression ignition engine is a diesel engine.

The vehicle may be a light-duty diesel vehicle (LDV), such as defined inUS or European legislation. A light-duty diesel vehicle typically has aweight of <2840 kg, more preferably a weight of <2610 kg.

In the US, a light-duty diesel vehicle (LDV) refers to a diesel vehiclehaving a gross weight of ≦8,500 pounds (US lbs). In Europe, the termlight-duty diesel vehicle (LDV) refers to (i) passenger vehiclescomprising no more than eight seats in addition to the driver's seat andhaving a maximum mass not exceeding 5 tonnes, and (ii) vehicles for thecarriage of goods having a maximum mass not exceeding 12 tonnes.

Alternatively, the vehicle may be a heavy-duty diesel vehicle (HDV),such as a diesel vehicle having a gross weight of >8,500 pounds (USlbs), as defined in US legislation.

The fourth aspect of the invention relates to a method of treating anexhaust gas from a compression ignition engine, which method comprisescontacting the exhaust gas with an oxidation catalyst, wherein theoxidation catalyst comprises: a platinum group metal (PGM) componentselected from the group consisting of a platinum (Pt) component, apalladium (Pd) component and a combination thereof; an alkaline earthmetal component; a support material comprising a modified aluminaincorporating a heteroatom component; and a substrate, wherein theplatinum group metal (PGM) component, the alkaline earth metal componentand the support material are disposed on the substrate.

Typically, the method involves contacting the exhaust gas directly fromthe compression ignition engine with the oxidation catalyst. Thus, it ispreferred that additional hydrocarbon (HC) is generally not injectedinto the exhaust gas prior to contacting the exhaust gas with theoxidation catalyst. The amount of hydrocarbon in the exhaust gas ispreferably less than 1,000 ppm by volume, as converted to methane, morepreferably less than 950 ppm by volume, still more preferably less than750 ppm, typically before contacting the exhaust gas with the oxidationcatalyst.

A fifth aspect of the invention relates to a process of preparing theoxidation catalyst. The invention also relates to an oxidation catalystobtained or obtainable by the process. The process comprises disposing aplatinum group metal (PGM) component, an alkaline earth metal component,and a support material onto a substrate, wherein the platinum groupmetal (PGM) component is selected from the group consisting of aplatinum (Pt) component, a palladium (Pd) component and a combinationthereof, and the support material comprises a modified aluminaincorporating a heteroatom component.

Each layer/zone of the catalyst may be formed on the substrate byapplying a washcoat coating to the substrate. Generally, each washcoatcoating is an aqueous dispersion that includes each of the componentsthat make up the relevant layer of the catalyst. The components thatmake-up the washcoat coating can be pre-formed.

Typically, the step of disposing a platinum group metal (PGM) component,an alkaline earth metal component, and a support material onto asubstrate involves applying a washcoat coating to the substrate to forma coated substrate. When the oxidation catalyst comprises a plurality oflayers, then the process involves applying a plurality of washcoatcoatings onto the substrate to form a plurality of layers. Afterapplying the or each washcoat coating, or after applying the pluralityof washcoat coatings, a step of calcining the coated substrate attemperature of 400 to 800° C., preferably 450 to 600° C., is generallyperformed.

For an oxidation catalyst comprising two layers, the step of disposing aplatinum group metal (PGM) component, an alkaline earth metal component,and a support material onto a substrate involves applying a firstwashcoat coating to the substrate for forming a first layer, thenapplying a second washcoat coating to the substrate for forming a secondlayer. After applying at least the first washcoat coating, preferablyafter applying the first washcoat coating and the second washcoatcoating, a coated substrate is obtained. The process then involves astep of calcining the coated substrate at temperature of 400 to 800° C.,preferably 450 to 600° C., is typically performed. The first and secondcoatings form the first and second layers of the catalyst respectively.

Typically, each washcoat coating may independently comprise an aqueousdispersion of a platinum group metal (PGM) salt, a support material andoptionally an alkaline earth metal salt. During calcination, theplatinum group metal (PGM) salt forms the platinum group metal (PGM)component and the alkaline earth metal salt forms the alkaline earthmetal component.

Suitable platinum group metal (PGM) salts are well known in the art andinclude, for example, platinum nitrate and palladium nitrate. Examplesof suitable alkaline earth metal salts include an alkaline earth metalnitrate and an alkaline earth metal acetate. The amount and identity ofeach salt that is used is determined by the desired composition of thelayer that is to be formed.

The support material is selected from the group consisting of a secondsupport material as defined above, the support material comprising themodified alumina and a combination thereof. It is preferred that thesupport material is the support material comprising the modifiedalumina.

Each washcoat coating may optionally further comprise a noble metal saltand/or an oxygen storage material and/or a hydrocarbon adsorbent. It ispreferred that at least one, or each, washcoat coating further comprisesa hydrocarbon adsorbent.

A first embodiment of the process of preparing the oxidation catalystcomprises: applying a washcoat coating to a substrate to form a coatedsubstrate, wherein the washcoat coating comprises a platinum group metal(PGM) salt, an alkaline earth metal salt, a support material comprisingthe modified alumina and optionally a hydrocarbon adsorbent and/or anoxygen storage material and/or a noble metal salt; and calcining thecoated substrate at a temperature of 400 to 800° C., preferably 450 to600° C.

A second embodiment of the process of preparing the oxidation catalystcomprises: applying a first washcoat coating onto a substrate to form afirst coating; applying a second washcoat coating onto the substrate;and calcining the substrate, after applying the first coating and/or thesecond coating, at a temperature of 400 to 800° C., preferably 450 to600° C.

Typically, the first washcoat coating comprises a platinum group metal(PGM) salt, an alkaline earth metal salt, a support material andoptionally a hydrocarbon adsorbent and/or an oxygen storage materialand/or a noble metal salt and/or a second storage material. Thecomposition of the first washcoat coating being selected to form a firstlayer as defined above. The second washcoat coating comprises a platinumgroup metal (PGM) salt, an alkaline earth metal salt, a support materialand optionally a hydrocarbon adsorbent and/or an oxygen storage materialand/or a noble metal salt and/or a second storage material. Thecomposition of the second washcoat coating being selected to form asecond layer as defined above.

There are several methods of preparing a washcoat coating and often themethod used is determined by the catalyst layer that is intended to beformed.

One way of preparing the washcoat coating, preferably the first washcoatcoating and/or the second washcoat coating, is by dispersing a platinumgroup metal (PGM) salt, an alkaline earth metal salt, a support materialand optionally a hydrocarbon adsorbent and/or an oxygen storage materialand/or a noble metal salt and/or a second storage material in an aqueoussolution.

Another way of preparing the washcoat coating, preferably the firstwashcoat coating and/or the second washcoat coating, is by impregnatingthe support material with either an alkaline earth metal salt or analkaline earth metal component to form an impregnated support material,and then dispersing the impregnated support material, a platinum groupmetal (PGM) salt, and optionally a hydrocarbon adsorbent and/or anoxygen storage material and/or a noble metal salt and/or a secondstorage material in an aqueous solution. The step of impregnating thesupport material may involve either (a) spray drying an alkaline earthmetal salt or an alkaline earth metal component onto the supportmaterial or (b) dissolving an alkaline earth metal salt in an aqueoussolution, and then adding the support material to form a solutioncomprising the support material and the alkaline earth metal salt. Afteradding the support material in step (b), the solution comprising thesupport material and the alkaline earth metal salt is stirred and,preferably, the pH of the solution is adjusted to precipitate analkaline earth metal salt or component.

A further way of preparing the washcoat coating, preferably the firstwashcoat coating and/or the second washcoat coating, is by impregnatingthe support material with either a platinum group metal (PGM) salt toform a PGM impregnated support material, and then dispersing the PGMimpregnated support material, an alkaline earth metal salt, andoptionally a hydrocarbon adsorbent and/or an oxygen storage materialand/or a noble metal salt and/or a second storage material in an aqueoussolution. Methods for impregnating alumina support materials with a PGMare well known in the art.

Definitions

For the avoidance of doubt, the term “modified alumina incorporating aheteroatom component” does not embrace “pure” alumina (i.e. aluminahaving a purity of ≧99.9%), a mixture of alumina and the heteroatomcomponent, such as a mixture of silica and alumina, or a zeolite. In thecontext of the “modified alumina incorporating a heteroatom component”,any amount in % by weight refers to the amount of heteroatom component,whether an element, ion or a compound, that is present in the hostlattice of alumina with the remainder consisting essentially of alumina.

The term “alumina doped with a heteroatom component” generally refers toa material comprising a host lattice of alumina that is substitutiondoped or interstitially doped with a heteroatom component. In someinstances, small amounts of the heteroatom component may be present(i.e. as a dopant) at a surface of the alumina. However, most of thedopant will generally be present in the body of the host lattice of thealumina. Alumina doped with a heteroatom component is generallycommercially available, or can be prepared by conventional methods thatare well known in the art or by using a method as described in U.S. Pat.No. 5,045,519.

The term “alkaline earth metal component” as used herein generallyrefers to an element or ion from Group 2 of the Periodic Table, acompound comprising an element or ion from Group 2 of the PeriodicTable, or a metal alloy comprising an element from Group 2 of thePeriodic Table, unless otherwise specified. The term “alkaline earthmetal component” typically does not comprise or include the “modifiedalumina incorporating a heteroatom component”. The “alkaline earth metalcomponent” is not an “alumina doped with a heteroatom component” or an“alkaline earth metal aluminate” as described herein.

Generally, the “alkaline earth metal component” is (i) a compoundcomprising an alkaline earth metal, and/or (ii) a metal alloy comprisingan alkaline earth metal. In the compound comprising an alkaline earthmetal, the alkaline earth metal is typically present as a cation. Thecompound may, for example, be an alkaline earth metal oxide, an alkalineearth metal nitrate, an alkaline earth metal carbonate, or an alkalineearth metal hydroxide. In the metal alloy, the alkaline earth metal istypically present in elemental form (i.e. as a metal). The alkalineearth metal component is preferably a compound comprising an alkalineearth metal, more preferably a compound comprising a single alkalineearth metal.

The term “platinum group metal (PGM)” as used herein generally refers toplatinum or palladium, unless otherwise specified. For the avoidance ofdoubt, this term does not, in general, include rhodium.

The term “platinum group metal (PGM) component” as used herein refers toany moiety that comprises a platinum group metal (PGM), such aselemental PGM (e.g. a PGM metal), a PGM ion (e.g. a cation, such asPt²⁺), a compound comprising a PGM (e.g. a PGM salt or an oxide of aPGM) or an alloy comprising a PGM (e.g. a platinum-palladium alloy). Theterm “platinum (Pt) component” as used herein refers to any moiety thatcomprises platinum, such as elemental platinum (e.g. platinum metal), aplatinum ion (e.g. a cation, such as Pt²⁺), a compound of platinum (e.g.a platinum salt or an oxide of platinum) or an alloy comprising platinum(e.g. a platinum-palladium alloy). The term “palladium (Pd) component”as used herein refers to any moiety that comprises palladium, such aselemental palladium (e.g. palladium metal), a palladium ion (e.g. acation, such as Pd²⁺), a compound of palladium (e.g. a palladium salt oran oxide of palladium) or an alloy comprising palladium (e.g. aplatinum-palladium alloy).

The “platinum (Pt) component” is typically platinum metal or an alloycomprising platinum, particularly a platinum-palladium alloy.Preferably, the “platinum (Pt) component” is platinum metal.

The “palladium (Pd) component” is typically palladium metal, palladiumoxide or an alloy comprising palladium, particularly aplatinum-palladium alloy. Preferably, the “palladium (Pd) component” ispalladium metal.

The term “noble metal component” as used herein refers to any moietythat comprises a noble metal, such as an elemental form of the noblemetal, an ion of the noble metal, a compound of the noble metal or analloy comprising the noble metal (e.g. a noble metal-platinum alloy or anoble metal-palladium alloy). It is preferred that the “noble metalcomponent” is a noble metal itself (i.e. in elemental form) or an alloycomprising the noble metal. More preferably, the “noble metal component”is the noble metal itself (i.e. in elemental form).

Any reference to an amount of the “platinum group metal (PGM)component”, “platinum (Pt) component”, “palladium (Pd) component”,“alkaline earth metal component” or the “noble metal component” as usedherein generally refers to the amount of, respectively, PGM, platinum,palladium, alkaline earth metal or noble metal that is present. Thus,for example, if the “platinum group metal (PGM) component”, “platinum(Pt) component”, “palladium (Pd) component”, “alkaline earth metalcomponent” or the “noble metal component” is a compound comprising,respectively, a PGM, platinum, palladium, an alkaline earth metal or anoble metal, then the stated amount refers only to the total amount ofthe said metal that is present and does not include the other componentsof the compound.

Amounts given in units of g ft⁻³ or g in⁻³ generally relate to thevolume of the substrate that is used.

Any reference to an amount of a material in terms of % by weight, suchas for the amount of alkaline earth metal component, typically refers toa percentage of the overall weight of the layer/zone (e.g. washcoatcoating) comprising that material.

The term “substantially cover” as used herein refers to at least 90%coverage, preferably at least 95% coverage, more preferably at least 99%coverage, of the surface area of the first layer by the second layer.

The term “substantially uniform length” as used herein refers to thelength of layer that does not deviate by more than 10%, preferably doesnot deviate by more than 5%, more preferably does not deviate by morethan 1%, from the mean value of the length of the layer.

The expression “consisting essentially” as used herein limits the scopeof a feature to include the specified materials or steps, and any othermaterials or steps that do not materially affect the basiccharacteristics of that feature, such as for example minor impurities.The expression “consisting essentially of” embraces the expression“consisting of”.

The term “zone” as used herein refers to a washcoat region or layerhaving a length that is less than the total length of the substrate. Ingeneral, the “zone” has a substantially uniform length. The termnormally refers to the side-by-side arrangement of two or more washcoatregions or layers on the same substrate.

EXAMPLES

The invention will now be illustrated by the following non-limitingexamples.

Example 1

Preparative Methods

Samples containing an alkaline earth metal component and alumina dopedwith a heteroatom component as a support material were prepared asfollows.

Silica doped alumina powder was slurried in water and milled to a d₉₀<20micron. Barium acetate was added to the slurry followed by appropriateamounts of soluble platinum and palladium salts. The slurry was thenstirred to homogenise. The resulting washcoat was applied to acordierite flow through monolith having 400 cells per square inch usingestablished coating techniques. The part was dried and calcined at 500°C.

For comparative purposes, samples containing alumina doped with aheteroatom component as a support material, but without the alkalineearth metal component were also prepared. The method above was used toprepare the samples except that the step of adding barium acetate wasomitted.

As a further comparison, samples containing conventional alumina as asupport material with and without the alkaline earth metal componentwere prepared as follows.

Alumina powder was slurried in water and milled to a d₉₀<20 micron.Barium acetate was added to the slurry followed by appropriate amountsof soluble platinum and palladium salts. The slurry was then stirred tohomogenise. The resulting washcoat was applied to a cordierite flowthrough monolith having 400 cells per square inch using establishedcoating techniques. The part was dried and calcined at 500° C.

Analogous alumina samples without the alkali earth metal component wereprepared by omitting the barium acetate addition step.

The formulations contained a total loading of 50 gft⁻³ of platinum groupmetal. The samples containing barium were prepared using a loading of150 gft⁻³. Alumina doped with 5% silica was used as the modifiedalumina.

Measurement of CO T₅₀

The catalytic activity was determined using a synthetic gas benchactivity test (SCAT). Parts to be tested were first cored using a coredrill and aged in an oven at 750° C. for 5 hours using hydrothermalconditions (10% water). The aged cores were tested in a simulatedcatalyst activity testing (SCAT) gas apparatus using the inlet gasmixtures in Table 1. In each case the balance is nitrogen.

TABLE 1 CO 1500 ppm  HC (as C₁) 430 ppm NO 100 ppm CO₂ 4% H₂O 4% O₂ 14% Space velocity 55000/hourResults

The results of the measurements are shown in FIG. 1. FIG. 1 shows theimproved activity of catalysts of the invention, which comprise both amodified alumina and barium at a loading of 150 gft⁻³. The catalysts ofthe invention have a lower CO T₅₀ light off temperature than thecomparative catalysts that do not contain barium. This is particularlyapparent when the ratio by mass of Pt:Pd is in the range of 1:2 to 2:1.The catalysts containing conventional alumina as a support material andbarium do not show an improved CO T₅₀ light off temperature compared tocatalysts containing conventional alumina and no barium.

Example 2

Samples containing alumina doped with 5% silica, Pt:Pd in a mass ratioof 1:1 at a total PGM loading of 50 gft⁻³ and varying amounts of bariumwere prepared using the method described above. The CO T₅₀ light offtemperatures were also measured using the same procedure as set outabove.

Results

The results of the CO “light-off” measurements are shown in Table 2below.

TABLE 2 Sample No. Amount of Ba (gft⁻³) CO T₅₀ (° C.) 2-1 0 186 2-2 150170 2-3 300 166

Example 3

Preparative Method

Silica doped alumina powder was slurried in water and milled to a d₉₀<20micron. Strontium acetate was added to the slurry followed byappropriate amounts of soluble platinum and palladium salts. The massratio of Pt:Pd was 1:1 at a total PGM loading of 50 gft⁻³. The slurrywas then stirred to homogenise. The resulting washcoat was applied to acordierite flow through monolith having 400 cells per square inch usingestablished coating techniques. The part was dried and calcined at 500°C. The CO T₅₀ light off temperatures were also measured using the sameprocedure as set out above.

Results

The results of the CO “light-off” measurements are shown in Table 3below.

TABLE 3 Sample Alkaline Earth Amount of CO T₅₀ No. Metal (AEM) AEM(gft⁻³) Support (° C.) 3-1 Ba 100 A1 164 3-2 Ba 150 A2 166 3-3 Sr 150 A1178 3-4 Sr 300 A1 176

Catalysts containing strontium show a lower CO T₅₀ light off than acomparative catalyst without strontium (see Sample 2-1 in Table 2).Samples 3-1 and 3-2 show that a reduction in CO T₅₀ light off isachieved using two different hetero-atom doped aluminas. Support A1 is a5% silica doped alumina and support A2 is a 10% silica doped alumina.

Example 4

Preparative Method

Magnesium doped alumina powder was slurried in water and milled to ad₉₀<20 micron. Barium acetate was added to the slurry followed byappropriate amounts of soluble platinum and palladium salts. The massratio of Pt:Pd was 2:1 at a total PGM loading of 50 gft⁻³. The slurrywas then stirred to homogenise. The resulting washcoat was applied to acordierite flow through monolith having 400 cells per square inch usingestablished coating techniques. The part was dried and calcined at 500°C.

For comparative purposes, catalysts containing magnesium doped aluminaas a support material without an alkali earth metal were prepared in thesame way, except that the barium acetate addition step was omitted.

The CO T₅₀ light off temperatures were also measured using the procedureset out above.

Results

The results of the CO “light-off” measurements are shown in Table 4below.

TABLE 4 Sample No. Amount of Ba (gft⁻³) Support CO T₅₀ (° C.) 4-1 0 A1177 4-2 130 A1 170 4-3 0 A3 193 4-4 130 A3 174 A3 = Magnesium aluminatehaving 20% by weight magnesium.

The samples containing a support material comprising magnesium aluminateshow a lower CO T₅₀ light off when barium is included in the formulationthan those sample prepared without barium. The light off temperature isreduced by 19° C.

Example 5

Preparative Method

A catalyst (5-1) was prepared via incipient wetness impregnation ofbarium acetate solution on to a silica doped alumina support. Thematerial was dried at 105° C. A second solution of platinum andpalladium salts was then added by incipient wetness impregnation. Theresulting material was dried at 105° C. then calcined at 500° C. Thefinal composition was 0.65 wt % Pt, 0.35 wt % Pd and 10 wt % Ba.

A comparative catalyst (5-2) was prepared using the same method butwithout the barium acetate impregnation on to the silica doped aluminasupport. The final composition was 0.65 wt % Pt and 0.35 wt % Pd.

The % NO oxidation activity against temperature of each catalyst wastested when the catalyst was freshly prepared (e.g. “fresh” catalyst)and after hydrothermal ageing each catalyst at 750° C. for 48 hours(e.g. “aged” catalyst). The test gas mix is given in Table 5. In eachcase the balance is nitrogen.

TABLE 5 CO 1500 ppm  HC (as C₁) 783 ppm NO 100 ppm CO₂ 5% H₂O 5% O₂ 14% Results

The difference between the activity of the “fresh” and the “aged”versions of each catalyst is shown in Table 6 below.

TABLE 6 Difference in % NO oxidation between “fresh” and “aged” SampleNo. 210° C. 270° C. 5-1 3% 4% 5-2 21% 23%

The results in Table 6 show that catalyst 5-1 has a smaller differencein % NO oxidation performance between “fresh” and “aged” than catalyst5-2. This difference is important for exhaust system where there is adownstream emissions control device, particularly a SCR or SCRFcatalyst, because the activity of such downstream emissions controldevices may be affected by the NO_(x) content of the exhaust gas,especially the NO:NO₂ ratio.

For the avoidance of any doubt, the entire content of any and alldocuments cited herein is incorporated by reference into the presentapplication.

The invention claimed is:
 1. An oxidation catalyst for treating carbonmonoxide (CO) and hydrocarbons (HCs) in exhaust gas from the compressionignition engine, wherein the oxidation catalyst comprises: a substrate,which is a flow through monolith; a first layer disposed on thesubstrate, which first layer is a first zone; and a second layerdisposed on the substrate, which second layer is a second zone; whereinthe first zone or the second zone comprises: a platinum group metal(PGM) component selected from the group consisting of a platinum (Pt)component, a palladium (Pd) component and a combination thereof; analkaline earth metal component; a support material comprising a modifiedalumina incorporating a heteroatom component; and wherein the platinumgroup metal (PGM) component, the alkaline earth metal component and thesupport material are disposed on the substrate, and the total amount ofthe alkaline earth metal component is 10 to 500 gft⁻³.
 2. An oxidationcatalyst according to claim 1, wherein the modified aluminaincorporating a heteroatom component is an alumina doped with aheteroatom component, an alkaline earth metal aluminate or a mixturethereof.
 3. An oxidation catalyst according to claim 2, wherein theheteroatom component comprises silicon, magnesium, barium, lanthanum,cerium, titanium, zirconium or a combination of two or more thereof. 4.An oxidation catalyst according to claim 1, wherein the modified aluminaincorporating a heteroatom component is alumina doped with silica.
 5. Anoxidation catalyst system according to claim 1, wherein the modifiedalumina incorporating a heteroatom component is an alkaline earth metalaluminate.
 6. An oxidation catalyst according to claim 1, wherein thealkaline earth metal component comprises magnesium (Mg), calcium (Ca),strontium (Sr), barium (Ba) or a combination of two or more thereof. 7.An oxidation catalyst according to claim 6, wherein the alkaline earthmetal component comprises strontium (Sr) or barium (Ba).
 8. An oxidationcatalyst according to claim 1, wherein the first zone is disposeddirectly on the substrate.
 9. An oxidation catalyst according to claim1, wherein the second zone is disposed directly on the substrate.
 10. Anoxidation catalyst according to claim 1, wherein the second layer partlyoverlaps the first layer.
 11. An oxidation catalyst according to claim1, wherein first zone is upstream of the second zone.
 12. An oxidationcatalyst according to claim 1, wherein the second zone is upstream ofthe first zone.
 13. An oxidation catalyst according to claim 1, whereinthe first zone has a length of 10 to 80 % of the length of thesubstrate.
 14. An oxidation catalyst according to claim 1, wherein thesecond zone has a length of 10 to 80 % of the length of the substrate.15. An oxidation catalyst according to claim 1, wherein the first zonecomprises a platinum group metal (PGM) component consisting of acombination of a platinum (Pt) component and a palladium (Pd) component,and the second zone comprises a platinum group metal (PGM) componentconsisting of a combination of a palladium (Pd) component and a platinum(Pt) component, and wherein the ratio by mass of the platinum (Pt)component to the palladium (Pd) component in the first zone is differentto the ratio by mass of the platinum (Pt) component to the palladium(Pd) component in the second zone.
 16. An oxidation catalyst accordingto claim 1, wherein the first zone comprises a platinum group metal(PGM) component selected from the group consisting of a Pd component anda combination of a palladium (Pd) component and a platinum (Pt)component, and wherein the second zone comprises a platinum group metal(PGM) component consisting of a Pt component.
 17. An oxidation catalystaccording to claim 1, wherein the first zone comprises a platinum groupmetal (PGM) component selected from the group consisting of a platinum(Pt) component and a combination of a palladium (Pd) component and aplatinum (Pt) component, and the second zone comprises a platinum group(PGM) component consisting of a palladium (Pd) component.
 18. Anoxidation catalyst according to claim 1, wherein the first zonecomprises a platinum group (PGM) component selected from the groupconsisting of a platinum (Pt) component and a palladium (Pd) component,and the second zone comprises a platinum group metal (PGM) componentconsisting of a combination of a palladium (Pd) component and a platinum(Pt) component.
 19. An exhaust system for a compression ignition enginecomprising an oxidation catalyst according to claim 1 for treatingcarbon monoxide (CO) and hydrocarbons (HCs) in exhaust gas from thecompression ignition engine.
 20. A method of treating carbon monoxide(CO) and hydrocarbons (HCs) in an exhaust gas from a compressionignition engine, which method comprises contacting the exhaust gas withan oxidation catalyst, wherein the oxidation catalyst comprises: asubstrate, which is a flow through monolith; a first layer disposed onthe substrate, which first layer is a first zone; and a second layerdisposed on the substrate, which second layer is a second zone; whereinthe first zone or the second zone comprises: a platinum group metal(PGM) component selected from the group consisting of a platinum (Pt)component, a palladium (Pd) component and a combination thereof; analkaline earth metal component; a support material comprising a modifiedalumina incorporating a heteroatom component; and wherein the platinumgroup metal (PGM) component, the alkaline earth metal component and thesupport material are disposed on the substrate, and the total amount ofthe alkaline earth metal component is 10 to 500 gft ⁻³.